Memory error from imagefill()

Might just be my issue, but I get a memory error when submitting a node with a signature field:
Fatal error: Allowed memory size of 67108864 bytes exhausted (tried to allocate 28512001 bytes) in /home/mysite/public_html/sites/all/modules/signaturefield/signaturefield.module on line 25

Line 25 is imagefill($src_image, 0, 0, $bg);.

Could my PHP install have anything to do with it?