I have a number of images that have all the **same background** but each has **different text** overlaid (they are covers of a magazine journal). 

I don't have access to the background image, but I know that I can **create a composite of the covers and get the background that way.**

I can do this manually with a photo-editing program by moving from layer to layer but I'm wondering if there's a tool or filter that **automates** this (ideally one I can use in Gimp, since I don't have Photoshop).