I'm using AutoCAD 2019 but it's not the age of the program but the age of the menu.
Let me give you some background: I first developed this menu 30 years ago in AutoCAD Release 9 with the "MNU" format and it's been mostly working fine with a few tweeks at some version releases. For me the MNU format was easier to work with and I didn't have much need or time to get into the CUI format until now. Let's just say I'm comparatively new to the CUIx format. The BMP icons are not really a problem except I wanted to upgrade the look and add some new and remove some old commands that are not used anymore.
So I have started to change some icons to PNG images. I used the term "BMP" generically as the image format isn't the problem I've been having. The problem is, I create the image and assign it to a command. Then if it doesn't look quite right I modify it and save it, but it doesn't update in AutoCAD. I find that I have to save to a different name and load the file with the new name and repeat until the icon looks good. OK, but the old files were still there in the CUI file. So I searched the forums for "Icon Manager" with not much for results so I started this thread.
Then, on my own I stumbled on the "Image Manager". The manager has 3 functions. "Import", "Export", & "Delete". The file list shows which files are not referenced and the delete function works fine so that problem is solved.
If you Import a file with the same name into the file, it does not replace the old file. It doesn't do anything really. I tried your "ZIP" file solution and it works but is a little "clunky" so I'll do that until I find a better solution. I guess this is solved also.
Thanks!