Copy from clipboard using Python and Tkinter
Using Tkinter, I would use a try..except block to insert the clipboard data where it exists and ignore it where it doesn't (or, optionally, add some default value). This doesn't specifically revoke any image-type clipboard contents, but it will reject anything that isn't in myTkObject.clipboard_get()'s expected format. That's a string by default, though you can change it with the function's type keyword argument.
Here's an example that builds on Nodak and jonrsharpe's answer and comments:
from Tkinter import TkmyTkObject = Tk()try: listbox.insert(0, myTkObject.clipboard_get())except Tkinter.TclError: pass # Instead of do-nothing, you can insert some default value if you like.