GNU Image Manipulation Program (GIMP)

$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

source: 適用於Ubuntu的Photoshop替代方案 – Linux Photoshop替代方案