Same problem for me, using a heavily modified creload 6.15
i've done the test http://shop/validation_png.php?rsid=XXXXXXXXXX and seen that the image was correctly generated
The problem for me was in the tep_image function, that you can find in catalog/include/functions/html_output.php.
That function was previously modified foroption of image size, that modification was not correct with this contribution.
my code was :
hope this helps,