GIMP
The Free & Open Source Image Editor
GIMP is a cross-platform image editor available for GNU/Linux, OS X, Windows and more operating systems. It is free software, you can change its source code and distribute your changes.

Whether you are a graphic designer, photographer, illustrator, or scientist, GIMP provides you with sophisticated tools to get your job done. You can further enhance your productivity with GIMP thanks to many customization options and 3rd party plugins.
Quellen & weiterführende Infoshttps://www.gimp.org/
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GIMP Basics

Gimp bietet zahlreiche Tutorials (Deutsch, Englisch,...) an.

Um Basics zu lernen oder aufzufrischen eignet sich zum Beispiel die Tutorials Nick Saporito, Graphic Designer & Content Creato:


Starte mit diesem Video zu den Werkzeugen von GIMP:

https://logosbynick.com/gimp-tools-explained/#mypaint-brush


All 39 GIMP Tools Explained 
Move
Alignment
Rectangle Select
Ellipse Select
Free Select
Scissors Select
Foreground Select
Fuzzy Select
Select By Color
Cropping
Rotate
Scale
Shear
Flip
Perspective
Unified Transform
3D Transform
Handle Transform
Warp Transform
Cage Transform
Bucket Fill
Gradient
Paintbrush
Pencil
Airbrush
Ink Tool
MyPaint Brush
Eraser
Clone
Perspective Clone
Healing
Smudge
Blur & Sharpen
Dodge & Burn
Paths
Text
Color Picker
Measure
Zoom
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