Your question said "a" site, but I assume you mean "this" site? If so, use the "Upload Images" link on the left side of the page and fill in the blanks.
"Image Ttile" is where you type the name of your image.
"Description" is where you can describe details about the image.
Use the "Browse" button on that page to choose the original image from your computer that you'd like to upload (no larger than 1600x1200).
Then select the appropriate "Gallery" that your image belongs in.