Having the same problem in [vanilla] AutoCAD 2012 as well:
I have several custom toolbars with buttons and some of the images are not showing up in the CUI editor for me to edit/use/see.
I have the images on the server on a dedicated drive and the path is 3rd from the top on the files tab - SFSP (all of the C:\ProgramFiles\AutoCAD.... are below it) in options.
The odd thing is that I can see some of the custom images in the Button Editor on the right side of the CUI Editor, but not all, especially the newer ones I have been creating lately. I can create and save them to the network location, but cannot see them to edit nor assign them to a button within the editor.
Is there something I am forgetting to do ?
Thanks in advance for any ideas.