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.

For Full jpeg resize go here: http://www.jpegwizard.com/

For an excellent (free) image-handling program I recommend IrfanView
http://irfanview.com
Use Image > Resize/Resample to make the correct size thumb then File > Save as...newname_tn.jpg.
You will have a slider that allows you to compress the image.
Try a setting of 80 to start and adjust as needed.

Use Internet Explorer for uploads – Netscape changes the default thumbnail
size and handles scripting poorly.