This actually happens when a program makes itself the default program with all the files and then something else messes that up. It's NOT ImgBurn doing it.

ImgBurn is obviously not interested in zip files but it might have had to add an entry to a more generic key so that it's own 'Burn using ImgBurn' entry would be visible on the context menu for the files it IS interested in.

i.e you might see the following in regedit under HKCR:

'.iso' -> default shell extension 'SomeProgram.Image'
'.img' -> default shell extension 'SomeProgram.Image'
'.zip' -> default shell extension = 'SomeProgram.Image'
'SomeProgram.Image' -> details of how to open that program.

ImgBurn would have to hook into the 'SomeProgram.Image' one. It would NOT touch the '.zip'.

The mess was caused by another program and you're the only person that can find out which one that is.