I'm both puzzled and ticked off at trying to post to my gallery.  It keeps rejecting my thumbnail dispite it being 180X193 and just over 12k.  You state that the thumbnail must be 100X100 minimum and 200 X 200 maximum and not bigger than 15k.  last time I checked 180 X 193 is between the minimum and maximum and the file size is less than 15k.  The only thing I can think of that may be causing my problem is that the info you give for posting is WRONG and that the thumbnail MUST be either 100 X 100 or 200 X 200.
Suggestion:  You could save people a lot of aggravation by correcting the upload information on the upload form.