Thankyou for your quick reply.
I cannot remember exactly.  I think it was along the lines of telling me to submit the correct size. So I though "I will try not submitting one"  as it previously suggested when folk were having difficulty getting the thumb size right. ...and it worked- a thumb appeared in the fractal gallery. So -I just wondered if it were worth bothering trying to get the size right if one was going to be automaticallly generated.