My Canon 450D gives an aspect ratio of 3/2 (4272 x 2848 pixels) which makes it difficult to provide 1600x 1200 images for uploading here.
Is there any way to change this to 4/3 in camera or is post cropping the only way ?

There is no way to change the aspect ratio in camera Eve. If you are using Photoshop you can change the crop too to 4/3 ratio or any other ratio for matter. With the crop tool selected, click the style button and change it to Fixed Aspect Ratio then dial in the ratio you want.

Thanks Robert - I dont use PS much but in PSP I can choose Canvas Size and go from there. I just hate losing part of the image :)
I have been uploading some 3/2 sizes but prefer to do a standard size.

I um goofed. The tool to use in Photoshop is the square marquee tool. Occasionally you have to listen to what I mean rather than what I say.

Hey Eve,
I'm not sure what aspect my Pentax produces but knowing I virtually always crop my images for the aspect of my 1680x1050 screen I tend to keep that in mind as I'm taking photos, in my case it means usually losing something off the top or bottom and I take my photos accordingly.
Mikel.

Thanks Mikel - that sounds like a good idea.
I became so used to 4/3. It's hard to teach an old dog new tricks lol.