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24 of the best free alternatives to the most popular paid software
(www.xda-developers.com)
This is a most excellent place for technology news and articles.
If you're working with RAW files, neither Gimp nor Krita are the tool for you.
I use Darktable to adjust global things like brightness, contrast, white balance and so on, then export to Krita or Gimp for more granular local modifications.