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, colour and shape selector are on the left hand side. Other more complex tools and filters such as Raindrops are in the top menus.