I first came across the Imagemagick library and toolkit when I was new to Linux and trying to satisfy Enlightenment and E-term dependencies and get them up and running. A task, and this was the mid 90s, I do not recall fondly. (And thinking about that experience, despite all the controversy surrounding Gnome 3, and Unity, the Linux Desktop has come a long way).
Anyway, unlike either Enlightenment or E-Term, Imagemagick has been a firm friend ever since.
There are three main tools I find I fall back on, time and time again.
Display, Convert, and Identify.
Display, funnily enough, will open up an x-window with the contents of an image.
Convert, perhaps the most useful tool of them all, lets you resize, transform, and re-format images.
And lastly, identify, lets you get information about a file, size, format, colour depth, offset and so forth.
The power of these tools is best wielded with simple loops in bash, or using GNU Find. For example to convert a bunch of images from 1920x1080 to say 1280x720, you could do something like this
$ for IMAGE in *.jpg; do
> # usage convert <action> orgfile.jpg newfile.jpg
> convert -resize '1280x720' $IMAGE $(echo $IMAGE | sed 's/.jpg$/-resized.jpg/')
> done
Convert tries to be clever and maintain aspect ratios. In this case, 1920x1080 fits into 1280x720 maintaning a 16:9 aspect ratio. But what if we had a 1920x1200 input image? In these cases, convert would actually resize the image to something like 1280x800. But if we really want it to ignore common sense and squish things down to our requested 1280x720 you need to use the bang operator e.g.
$ convert -resize '1280x720!' srcimg.jpg dstimg.jpg