Peruse
Peruse is a free multi-platform open source software for reading books and comics from KDE
KOReader
KOReader an ebook reader application supporting PDF, DjVu, EPUB, FB2 and many more formats, running on Cervantes, Kindle, Kobo, PocketBook and Android devices
Calibre
Calibre is a free, open source cross-platform software for reading, converting, and organizing e-book collections
OpenComic
OpenComic – free and open source cross platform program for reading comics and manga
Komikku
Komikku – online/offline manga reader for GNOME, developed with the aim of being used with the Librem 5 phone
Qimgv
Qimgv – free and open source fast, configurable, easy to use image viewer with optional video support for Linux and Windows
Rapid Photo Downloader
Rapid Photo Downloader is a free and open source software for downloading photos and videos for the Linux Desktop
RawTherapee
RawTherapee is a free and open source, cross-platform raw image processing program
Darktable
Darktable – free and open source cross-platform software for processing and cataloging RAW images