What's the difference between a Bitmap and a JPEG, if there is any?
And what do people prefer to use, is there any advantage of one over the other? ....................and finally............are there any other file types which should be considered?

Well, bitmaps usually have the highest quality and are the largest files. JPEG is a compressed picture file. JPEG usually can become very small compared to a bitmap, but it can lessen quality depending on how compressed it becomes. Other picture file types include PNG, PCX, ICO, GIF....and others. As far as preference goes, it really depends. JPEG and GIF are generally used on the web because it is much smaller than say a BITMAP, and takes up much less space and bandwidth. However, I upload my pictures as large bitmaps, and the server on Caedes converts it to JPEG. If I were to convert it to JPEG myself and then upload it, it might make the quality lessen twice. It all depends on what you are looking for. My desktop wallapapers are bitmaps for the best quality. Web images are almost always in JPEG or GIF because of the less ammount of space they take up. I hope I answered your question :)

Thanks for taking the time to answer, it was spot on. I just had no idea why I always used JPEG's!! Maybe you can help further.............
If then, when converting from Bitmap to JPEG the image gets compressed, is there anyway of determining how much compression is applied?

compare the two file sizes.

Well, some programs that convert BMP to JPEG allow you to set the quality. Generally, the lower the quality, the smaller the size. If you're using the good ol' MSPaint, all you can really do is save as extension .JPG and compare the sizes of them later.

Does photoshop have the ability to set the quality?

Yes
Save As...>> select from the dropdown to JPEG
Save
Another window will pop up, there you can set the quality. You'll see.

It's the bit I've always breezed past and just pressed OK..
Thanks for the help, cheers....

This seems to answer my frustration of why some of my photos seem lower quality once uploaded since I change from bitmap to jpeg. At least I know why now. Thanks Radjehuty for the lesson. Cheers Al.

you can tell how much compression is applied as caedes said by comparing the two file sizes, what he meant was dividing the difference between the file sizes by the new size and multiplying by 100. For example bitmap = 12k, jpeg =9k, 3/12=0.25 = 25% compression.

A recent image of mine had a file size of 5.49MB (Bitmap) and after converting to JPEG it was reduced to 1MB without any extreme noticable loss in quality. This is around 81% compression!!!!! (if my maths is right), does anyone know how compression works and why some images seem to be affected by a loss in quality more than others?

The amount of colors in a photo would have a great deal of affect on file size, for instance an all black photo would have an extremely small size, try it. The amount of small details and sharpness of a photo may also be affected by changes in compression.

It also has to do with what bit pixels they are. For instance, an image with 32-bit color as opposed to an image with 16-bit color, or even 256/16 color image. Basically, the less colors, the smaller the pic. Millions of colors deffinately takes up more space than 16 colors! :D

I believe we basically said the same thing accept Radjehurty was reffering to bitmap color depth in particular.

JPEG compression uses various algorithms, depending on the program and quality preferences. As you may know, everything on your computer is reduced to binary code, just zeros and ones. In a bitmap with 32-bit color, each pixel in the image has 32 zeros and ones to determine its color. In a 1600x1200 image that means 1600x1200x32 zeros and ones worth of color data. That is a lot of zeros and ones. Since there is no compression, you get maximum quality (and filesize).

With a jpeg, the compression algorithms look for patterns, large blocks of the same color, that it can reduce to a simplified code, that a jpeg decoder on some other machine will understand and be able to use to reconstruct the image (rough example, if it comes across 1000 zeros in a row, it could have a shorter code that tells the decoder to put 1000 zeros here when displaying, instead of saving all 1000 zeros in the file). Therefore, as was said by the others, images with a lot of one color and less small detail will benefit much more from compression than images that are harder to "summarize" due to color and detail. Then, the higher compression factor you allow, the more errors the algorithm will allow. This will enable it to compress colors and details more, but at some point the loss in quality becomes noticeable. If you have infinite amount of drive space you'd want to keep everything as a bitmap. I have nowhere near infinite drive space so I tend to use a low compression factor of about 2 most of the time in hopes of maintaining decent quality.

Anyway that's kind of a simplification of my understanding of how it works benroy, hope that answers your question. Compression algorithms are still being developed, particularly with all the audio and video compression being used that works in a similar way (mp3, mpeg, etc.). It's like building a better mousetrap now.

question asked, question answered

Thanks for that Torque, it explains very well how that image was compressed so much without losing much quality, and thanks to everyone else for sharing their knowledge.
Cheers.......

that does explain a lot. always wondered about that. thanks for taking the time guys. =)

personally i prefer JPG files just because that keeps everything standard on my machine (along with PSD's) i just always use minimum compression in photoshop, so usually the file is still fairly large ~half a bitmap size but i still keep full quality as far as anyone could tell, however if i'm still working on a picture, i'll always use PSD which saves at least as much info as a bitmap +some photshop info. you never want to save a working copy as JPG because then after opening and saving several times you will lose noticable quality.

Oh yes I forgot to mention that. I always use "save as" paintshop pro image, and "save copy as" a .jpg when it's ready to go or just to have versions to compare. If you keep resaving something as a .jpg you will definitely begin to see a loss in quality as Carl mentioned. If you happen to use paintshop pro, I find that it begins to create a grid of off-color dots all across the image from repeated compression. Very bad. I usually have lots of layers going on an image so I have to save it as a .psimage anyway :)

If I'm working on something that doesn't require layers then I save it as PNG. I compresses (unlike bitmap files) and is lossless. The main reason to not save as PSD all the time is that when working with large files (for print work) the files can get prohibitivly large when saved as PSD.