I imagine you are using GD to create thumbnails?
FAQ - Bad thumbnail quality with GD
You have to alter the constants.php file in your 4images/includes folder. Essentially, download that file to your computer, then edit it with a basic program (sorry, I don't know what it's called on a Mac). Once you finish editing, then upload it back to the same folder.
Here is my 4Images page with the thumbnail correction. I never tried creating thumbnails before altering the constants.php file, so I don't know what they could've looked like.
Cheers,
Lawrence