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.

Just curious, because I really don't know anything about creating custom thumbs because I always use just an autogenerated smaller version of my full image, but are you posting it in .jpeg?  I know Renderosity is not compatible with gif or bmp.

I owe an appology to this forum.  It seems that my photo program no matter how you change the dimentions changing the image size forgets all image dimention changes when you "save for web".  The dimentions of any cropping stay but the changes in image size have to be respecified. 
At the point when I posted I had tried 10 times and every time I tried to post the form resets the image and thumbnail names.

There's a serious suggestion for this forum.  When the thumbnail, or other items on the form are incorrect it would be more than useful if ONLY the area of the form in error were reset.  As it is now if you forget to check the item saying that this is your work of art it resets all of the form.  If it has a problem with the thumbnail it forgets which picture you wanted to upload.  Since the other information on the form is correct resetting the whole form should NEVER be done.

I could use a generated thumbnail in this case because the picture required a nudity tag and therefore a custom thumbnail.

Thanks for the suggestion, it will be added to the list. :)

I have had the same problem with uploads. Now I make them so small people complain because they cannot see the details of them.  I mainly upload in Photography.  There is definitely something wrong with Renderosity definitions of file sizes, in my opinion.