The GIMP is a wonderful image editor, but it might be overkill if all you want to do is annotate an image. If you want to highlight a part of an image, so that for example the audience for your presentation can focus on a particular aspect, you'll probably find it easier and more intuitive to do that in a program such as Dia or OpenOffice Draw. Let's see how to annotate an image in all three programs.
As an example, we'll highlight an image by circling an area and creating an arrow to point at the circle. All three of the tools mentioned allow you to work in different layers, separating the edits of the background from the annotations. We'll work with an image of coats of arms that dates from 1876, and highlight the coat of arms for Siam (Thailand), which shows an elephant. You could ask an audience to count down four rows and over two columns, but if you draw a circle around the item, the audience can quickly find it immediately.
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