I have been searching and tried everything I could find but haven't been able to fix this problem. Any suggestion would be greatly appreciated.

Via the product admin I uploaded the image for a product.
/store/images/abc/xyz.gif

Zencart has automatically created
/store/images/abc/xyz.medium.gif

I have uploaded via FTP
/store/images/medium/abc/xyz_MED.gif
/store/images/large/abc/xyz_LRG.gif

On the product page xyz.medium.gif is shown. The link to the large image displays xyz.gif

Can someone tell me why the _MED and _LRG images aren't being used?

Many Thanks