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.

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.