GIMP allows you to combine multiple images using layers, and to blend everything together using the color picker tool to match the background colors.

The Digital Photography School blog has an instructive tutorial on using layers for those just getting their feet wet inside Photoshop, or other high-end image editors like the open-source GIMP ...

New features in GIMP 3 include non-destructive filters, improved text editing, and simpler layer management. Working with filters, layers, and text is much easier in this latest release.