GIMP is the GNU Image Manipulation Program; a powerful application for raster image authoring, retouching and composition. GIMP is a scriptable, expandable, extendable image application with a nice GUI.
GIMP can open, display, convert and save to all sorts of image file formats, including BMP, GIF, JPG, PCX, PDF, PNG, PS, PSD, SVG, TIF and more. GIMP features painting tools (Airbrush, Brush, Clone, Pencil) including gradients, blending, custom brushes and patterns, anti-aliasing, and more.
GIMP has support for animation, multiple undo and redo, layers (incl. editable text layers) and channels (incl. alpha channel support), transformable paths, a foreground extraction tool, transformations (incl. flip, rotate, scale and shear) and extractions. GIMP is plug-in friendly; there are well over 100 plug-ins available.
More info: GIMP.org, User Manual, FAQ, Tutorials. Also check out Complete Intro to GIMP 2017 [videos], FLOSS Manual, GIMPology, GIMP tips and tricks, Getting Started with Gimp 2.8 [videos] and How to GIMP Tutorials.
gThumb is free and open source software licensed under the GPL. Learn more about gThumb on it's SourceForge home. See a (short) About.com gThumb Tutorial, see How to extend GIMP with GMIC on HowToForge, How to Perform Batch Resize of Images Using gThumb in Linux on MakeTechEasier, read the official Ubuntu gThumb Documentation, read an OpenSourcePhotography.org gThumb review and watch a gThumb Overview on YouTube and a video about gThumb from UbuntuAppWorld on YouTube.
Inkscape is free, open source software licensed under GPL. Learn more at the Inkscape Homepage and be sure to check out the unofficial user manual. Also see 20 Tutorials for Creating Amazing Graphics with Inkscape, Tech Drive-in's Beautiful Inkscape Tutorials, the Inkscape FLOSS Manual, the Inkscape Forum, Inkscape Tutorials, inkscape tutorials blog and the Inkscape Ubuntu Wiki.
LibreOffice-Draw is free, open source software licensed under the LGPL. BVCC Schoolhouse Draw Tutorials (bottom of page) in PDF format. See Video Tutorials from FrugalComputerGuy and the GoFree Tutorials. Also check out the LibreOffice-Draw official Homepage.
mtPaint is free, open source software licensed under GPL. Learn more at mtPaint's SourceForge Homepage. Learn much more from Lubuntu - MtPaint on YouTube, Lubuntu Screencast: MtPaint create wallpaper, Mark Tyler's own tutorials, the mtPaint Blog and The mtPaint Handbook.
Featuring precision text and image placement (including rotation), text kerning, layering with CMYK (and RGB) colors, spot color support, ICC color management, bezier curves polygons, PDF creation, encapsulated PostScript, EPS/PS and SVG import and export and press-ready 4-color separations.
More info at Scribus.net, and the Scribus Wiki. Read Getting Started with Scribus at Open-of-Course and check Scribus articles in LinuxJournal. See a newsletter-creation 3-video series at DecafBad, see the Scribus FLOSS Manual, watch Kevin Pugh's Videos and a good Scribus Video Tutorials collection on YouTube.
SCROT is a simple command-line utility for taking screenshots. The simplest way to utilize SCROT is by hitting the "Print Screen" button on the keyboard. An image of the entire screen will appear in the home folder. Many commands may be issued in LXTerminal to control SCROT including image format, time-delay, a particular part of the screen, etc. For many of SCROT's useful commands, just type:
man scrotin LXTerminal.
See Gary Richmond's "How to take screenshots" in Free Software Magazine, How to take screenshots on Linux using Scrot from HowToForge and Shambhu Singh's "Scrot: A Command Line Tool to Take Desktop/Server Screenshots Automatically in Linux" on Tecmint.
GNOME Screenshot is free and open source software licensed under the GPL. See A comprehensive guide to taking screenshots in Linux using gnome-screenshot on HowToForge, GNOME Screenshot info from Computers4Christians, the GNOME Screenshot Ubuntu Manpage and The Geek Stuff article Methods to Take Screenshot Capture in Ubuntu Linux.
Dia Diagram Editor for creating relationship and network diagrams, flowcharts, etc - Export to EPS, PNG, SVG, XFIG and WMF. FreeCAD is a 3D CAD/MCAD/CAx/CAE/PLM modeler. Gcolor2 is a color picker. Pencil is drawing software to create traditional 2D animations (cartoons) and supports bitmap and vector graphics. Other Paint Programs include Pinta and Krita. Other Photo Apps include F-Spot, GimPhoto, RawTherapee, Shotwell and digiKam.