Turning off automatic thumbnails.

[size=4][font=Arial]Is there a way to not use the thumbnail settings for products? if I leave the sizes blank for category images theirs no problem, but if I leave the sizes blank for products such as products list, products cart etc. I receive an error message when my products are displayed. Cs people suggested using CT after inndex.php? to clear the thumbnail cache. It does clear the cache but doesn't fix the problem…[/font]

[font=Arial]Any suggestions?[/font][/size]