Install GNU Image Manipulation Program (GIMP)
 

Ubuntu

sudo apt update
sudo apt upgrade
sudo apt install snapd
sudo snap install gimp

or

sudo apt update
sudo apt install gimp

Krita

sudo add-apt-repository ppa:kritalime/ppa
sudo apt-get update
sudo apt-get install krita

Darktable

sudo add-apt-repository ppa:pmjdebruijn/darktable-release
sudo apt-get update
sudo apt-get install darktable

digiKam

sudo add-apt-repository ppa:philip5/extra
sudo apt update
sudo apt install digikam5

RawTherapee

sudo add-apt-repository ppa:dhor/myway
sudo apt-get update
sudo apt-get install rawtherapee