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 ...

Launch GIMP and open the image that you want to make semi-transparent. Press "Ctrl-L" to display the Layers toolbox at the right of the GIMP window.

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.