ImageKlebor comes in two flavors: the graphical (GUI) version lets you put together your images in a visual (manual) way, while the command-line version is especially useful for pipeline integration and automated tasks.
ImageKlebor is available for Windows, Linux and MacOSX. The Windows version comes as a setup, the Linux version comes as an archive that simply needs to be extracted to a location of your choice, the MacOSX version comes as a disk image that can be easily installed into your /Applications folder. All versions contain both the graphical and the command-line version, several examples, as well as a comprehensive manual.
And best of all, ImageKlebor is completely free, even for commercial use.