Your CUIx file does not have the icon bitmaps in it. Change the .CUIx file extension to .zip. Insert your button images into the zip file, change the extension back to .CUIx. Reload the CUIx.
Since you are in the MEP section then i assume you probably mean the autocad and the MEP icons. This is how you extract them.
1. Go to the MEP main directory. In 2014 it is located at C:\Program Files\Autodesk\AutoCAD 2014\MEP.
2. Copy the acadbtn.xmx file from this directory to a new location. Change the file extension from .xmx to .dll.
3. Download Pixel Extractor.exe from this link. http://www.theswamp.org/index.php?topic=42619.0
4. Run Pixel Extractor and select the new named acadbtn.dll
5. Select all of the gorgeous Autodesk icons that it finds and right click and select Save as Another Image File.
6. Select a new location and file format to save the files to.
7. Repeat steps 2-7 for the file MEP.DLL
8. Open up the new file location, sit back and enjoy the 2,777 images that you are now the proud owner of to use in your own ribbons/toolbars
***BONUS*** As an added bonus, download a bulk rename utility and strip the extra information from the name of the files that you do not want.
*Edit* Added MEP.DLL to the image files to extract from.
Thanks Keith, but this all seems a bit too extravagant when AutoCAD had a fault and lost these on it's own. I didn't do anything to it, just opened AutoCAD MEP 2012 like I always do, and *pop* their all replaced with little clouds with ? marks in them.... To be honest, I'm a bit weary to run this procedure (besides, there is no "Pixel Extractor.exe". There has to be an easier way, short of uninstalling and re-installing......