codeigniter captcha Allowed memory

php.ini

Try changing these:

memory_limit=256M

For Forms / upload/download:
These two settings should be set to the same amount.

post_max_size=16M
and
upload_max_filesize=16M

Ther'isnt a problem of memory , this appear because ther's something  as a loop wich save image until the memory_limit are reached, the problem is why ?

show us the code.