GIMP - GIMP is a multiplatform photo manipulation tool.
draw.io - Online diagramming application
Adobe Photoshop - Adobe Photoshop is a webtop application for editing images and photos online.
LucidChart - LucidChart is the missing link in online productivity suites. LucidChart allows users to create, collaborate on, and publish attractive flowcharts and other diagrams from a web browser.
Krita - Krita is a professional FREE and open source painting program. It is made by artists that want to seaffordable art tools for everyone. Concept art. texture and matte painters, illustrations and comics.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.