It’s fairly easy to blend two images in GIMP. All you have to do is: Open the two Images in GIMP. Blend the images using layer opacity/blend mode/layer mask. Export the resultant image as JPEG/PNG.
We don't condone tagging (or whatever it is kids are calling it these days), nor would we want to explain to the local cop on duty or wandering gang banger why we're leaving our mark on a perfectly ...