Originals in PNG, thumbnails in JPG

[vlada]

Hello,
I made a gallery and I want to put some PNG images into it. The problem is, that even after resizing Coppermine keeps them as PNG. I don't think this is a good idea, because JPG is much more effective for most resized images. Can I somewhere set to use JPG compression for "normal" and "thumb" images?
I use GD library and Coppermine 1.3.2. I don't think my OS/PHP/MySQL version should have any influence.
Thank you,
Vlada

[vlada]

Now I found out it is different. I was surprised by a low size of the created .PNGs. So I checked them and I found out they are JPEGs with PNG extension.