Quote:... What I did not understand is why the writing does not appear in the thumb and therefore I was trying to figure out how to remove it..
It can depend where the thumbnail comes from. If it is embedded in the image, it can be a 'leftover' from before the image was edited, putting that banner at the bottom. If the thumbnail is separate and remains from a previous image, you might see that.
Thumbnails are in ~/.thumbnails/normal/ & maybe ~/.thumbnails/fail/gimp-2.8/
Back to the original question. The only way to remove the banner showing what was underneath is when the image is multi-layered, the banner being a layer. A gimp .xcf file or a PS .psd file.