You can exclude files from being mirrored based on their size (NOTE: filesize only, not pixel dimensions in the case of images).
Exclude all ZIP files larger than 1,000KB (1MB = 1,000KB) and all MPG files smaller than 100KB:
-*.zip*[>1000] -*.mpg*[<100]
Exclude thumbnail images (this is not a guaranteed exclusion—here we will just assume that the site's thumbnail filesize will be no larger than 20KB. You may have to adjust the values as needed):
-*.gif*[<20] -*.jpg*[<20]