You have 2 config.php files. One in your zencart dir /include/config.php and another in /admin/include/config.php.
The most likely the one in zencart/include/config.php will be mess-up if you get 404 durring checkout process. Compare both and edit the /include/config.php in your zencart dir.
I put zipped jpeg images using winrar.The program basically do not compree much. but give the file ext zip and the way can be downloaded intead of display on screen when buyer hit download button after susscesfull check out.
I use resized images for menu 800X600 and the contribution program called Image_Handler resize toicon and medium size. Works great.
zipped images goes to download dir and resized inserted durring the product adding.
Have a fun



