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.
Linux only (maybe): The free, open-source image editor, GIMP, uses multiple windows for its tools and dialogs, but they're getting combined into a single window in a future release. Impatient types ...