Limited software exists for designing photo collages on Linux. Depending on your needs, PhotoCollage and Fotowall can help you create appealing collages.
Photo collages can make websites more interesting or decorate walls as posters or framed pictures. All you need is the images that you want to appear in the photo mosaic and a software tool capable of making the composition.
In the Linux world, the choice of programs turns out to be quite small. Although there are now numerous online services that let you create collages, if you use them, you will disclose personal data that you can no longer control the use of. The recommendation is definitely to create photo collages on your own computer instead.
You can create photo mosaics using conventional image editing programs such as Gimp, but this requires a longer period of training. PhotoCollage and Fotowall, which are specifically designed for this purpose, make creating photo collages easier.
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