I found the file:
In this file all the text that is displayed within the UBB pop-up window is stored.
Now I can add more into it and will try whether the users will easier understand how to insert at least a link to an image...
I added now the UBB code for inserting an image:
Content visible to verified customers only.
I added 2 points within the bracket because otherwise it does not display the code. I think you had the same problem
Now my users at least see what the code looks like and they can try to use it. With this code, the image of the user, is still stored on his own website, but it appears immediately within the posting.
So this is not the ideal solution, but as long as PP does not offer a real image upload function within user's comments, this is a good compromise...