Some common photo editing software that comes pre-installed or can be installed and used on Ubuntu systems are:
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GIMP - Open source image editor, powerful, similar to Photoshop.
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Shotwell - Image viewing management software installed by default on Ubuntu, with basic editing functions.
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Pinta - Open source lightweight drawing and photo editing program.
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Inkscape - Well-known open-source vector graphics editor.
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Krita - Professional open source digital painting software.
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Darktable - Open source professional photo post-processing software.
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RawTherapee - Another open source RAW format image processing software.
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Pix - Simple and easy to use image editor.
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Phatch - Batch image processing tool.
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GThumb - Image browser, viewing, organizing tool.
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Fritzing - Image editor for circuit diagram design.
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MyPaint - Painting tools for digital graphics tablets.
In the software center, you can search and download tools suitable for different image processing needs. If you need more professional functions, GIMP and Krita are good choices.