Use Photoshop Save for Web. . .these settings are typical of most image editing apps.
Thumbnail size (weight) cannot exceed 15K, file dimensions must be less than 212 x 212
pixels.
If you don't have a compression program:
Try Thumbnail wizard here: http://www.jpegwizard.com/thumbn/thumbn.asp
For Full jpeg resize go here: http://www.jpegwizard.com/
For an excellent (free) image-handling program I recommend IrfanView
http://irfanview.com
Use Internet Explorer for uploads Netscape seems to try to change the default thumbnail
size and handles scripting poorly.