I use gimp, but it’s a steep learning curve and the construction of annotations is manual. gimp makes it hard work and I’ve not found a plugin that automates it. Same job can be scripted in ImageMagick, also much more effort that I care for!
A quick look at faststone seems to reveal a gimp-like process, but a notch more automated. But the user still has to create and position a bubble object, then overlay the bubble with a text object, and then add an arrow. (Might have been misled by an old example, and the latest version is slicker. Too many outdated reviews lurking on the web.)
Ideally, software should create entire annotations in a single easy step. Must exist, but I’ve failed to find an example. Anyone else?
Dave