Thread:Extension talk:UploadLocal/Images not getting thumbnails

I have MW 1.18 and am using the 1.19 snapshot (didn't see one for 1.18 and the trunk was empty). I applied the fix to line 31 of UploadLocalForm.php which now allows the uploads, however, the image thumbnails are not being generated... When I go to "http://mywiki/index.php/File:FILE-DSCN0109.JPG" I see all the metadata, and the full-resolution looks good, but the thumbnail is gray and says "Error Creating Thumbnail:"

Thank you!

edit: The thumbnails are not being generated for "large" JPG files (over 3 megs). I tried with lower resolution photos and those are working.... Ideas?