Afterburner contains a mixture of gif and png icons/images
If you have it set up as I have depicted it below, then the only thing stopping your custom images from working is that you will need to Purge all cache
After that you need to refresh your browser using keyboard Ctrl + F5 this forces all the images to reload.
As mentioned earlier...the pix_core and pix_plugins should be listed like so...
- yourthemename
- layout
- style
- pix
- pix_core
- pix_plugins
and NOT like this (just in case you have) they are NOT sub-directories of pix they are directories in their own right.
yourthemename
- layout
- style
- pix
- pix_core
- pix_plugins
Providing all the icons/images are in their respectful folders (look at Afterburner for guidance) and providing that you have not altered moodle/theme/image.php then it should work like Afterburner.
You could also try switching on Debugging in case there is something wrong with you theme?
Site Administration > Development > Debugging (set it to Developer settings)
Hope this helps?
Mary