You might have your programmer check CPAN (or google) for a Perl module called Image::Magick that is an integral part to several thumbnailing scripts that are available at places like hotscripts and cgi-resources. Image Magick can determine size, resize, and optimize images found in a directory or multiple directories, depending on how the script you use is configured.
You can also place size restrictions placed on the images they upload, both in byte size and image pixel dimensions if you can obtain those values during the upload process using a script. Images not meeting the necessary criteria are rejected and an error message is displayed to the person attempting to upload such images. This is something your programmer can help you with.