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, Documentation, User Manual, FAQ, Tutorials. Also check out tutorials from GIMPology and GIMP tips and tricks.
gThumb is free and open source software licensed under the GPL. Watch a UbuntuAppWorld video about gThumb on YouTube. Read a gThumb review on OpenSourcePhotography.org. Learn more about gThumb on it's SourceForge home. Read a gThumb review on Scribbles and Snaps. Read a (short) About.com gThumb Tutorial.
Inkscape is free, open source software licensed under GPL. Learn more at the Inkscape Homepage and be sure to check out their own user manual. Also see the inkscape tutorials blog, Speckyboy's Amazing Vector Graphics Tutorial and Tech Drive-in's Beautiful Inkscape Tutorials.
LibreOffice-Draw is free, open source software licensed under the Mozilla Public License v2.0. BVCC Schoolhouse Draw Tutorials (bottom of page) in PDF format. Watch FrugalComputerGuy's LibreOffice Draw Video Tutorials. Also check out the LibreOffice-Draw official Homepage
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, watch Kevin Pugh's videos and a good Scribus Videos 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. For tips and tricks see Gary Richmond's "How to take screenshots" in Free Software Magazine.
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.