Krita is the KDE version of GIMP or Photoshop. Actually, it is more than that. For the past ten years it has focused on cartoonists, illustrators, and concept artists in addition to image manipulation until today it is a painting tool designed for concept artists, illustrators, matte and texture artists, and the VFX industry.
You can still use it to easily manipulate images though with most of the simple options such as crop, color and shape selector are on the left hand side. Other more complex tools and filters such as Raindrops are in the top menus.
To install Krita on Kubuntu 20.04 open up Konsole and type,
sudo apt install krita
After installation Krita can get started either by searching in the Applications menu or by navigating to Applications/Graphis/Krita.