I think reasonable people can debate this one. setjmp()/longjmp() are very useful when working with libpng [1] and jpeglib [2], as you mentioned, as a crude exception handling mechanism. I have seen these functions used in production safely for that purpose. I can imagine other horrible uses for them though. Unlike things like strcpy() which are security-holes-by-design, setjmp()/longjmp() should be in a "carefully code review the exact way these are used" list rather than a banned list.
The trouble is how they compose with the expectations of code you don't write. It's just not normal in C to expect execution of your function to abort halfway through, and so memory leaks and broken state are almost guaranteed when they're mixed with the wrong third party library.
It might work great when you wrote it, but it might not even survive the next change to the codebase, and 10 years out, who is to know what programmers will come along and plug new stuff in without noticing the stack magic, etc
edit: another aspect of the same issue is that it can be difficult or impossible to write generic longjmp-safe code for many kinds of tasks. In C++ you have stack unwinding to e.g. delete large temporary heap allocations during a complex operation, but no such facility exists in C.
longjmp will not interrupt third party code (well maybe if it uses callbacks and you use it from a callback).
setjmp/longjmp will almost certainly require tracking any resource use carefully so that resources can be released at the setjmp point, when execution gets back there.
I mostly use it for terminating the recursive descent parsing on error. Otherwise it gets tedious to check return values everywhere in hand written code.
There is no reason libpng needed to use setjmp as part of its API, and that it does so is (I hope) widely regarded as a bad decision. It's "useful" only in so much as that is the API it defines, but there is no reason that this had to be the case, and simply returning an error code would have been very very much preferable.
1: http://www.libpng.org/pub/png/libpng.html 2: https://www.ijg.org