Image editing software
In this context, image editing includes selecting and saving regions of
interest, rotating, scaling, and annotation of images.
The FITS image viewer ds9 has all
the above facilities, with the exception of annotation.
The Gnu Image Manipulation Program, GIMP can do all of the above
but has a steep learning curve.
ImageMagick can also do all of the above and also has a
steep learning curve.
gnome-screenshot is remarkably useful for
cutting out and saving an area of an image displayed on the screen.
xpaint is a very
simple application I often use to annotate images with cross-hairs, arrows, object names, etc.