That's the source code, it's mostly done but it needs some very important touch-ups before it's ready for anyone to use. I made it for my game, since there really wasn't a good alternative. I should be able to get it working with Unity eventually, too.
You have to go to Add -> Add Image to start. You might notice the frame selector is pretty unfinished, I'm in the middle of adding functionality to swap limbs.
To add a keyframe drag and drop the image wherever, then click somewhere else in the frame selector (the grey area at the bottom) and drag it somewhere else. You should see it play the animation you just made with, with generated i-frames in between. Currently there has to be at least two keyframes for anything to work, it broke when I was working on something else.
Also right click hold to pan.
Soon enough I'll add curved and smoothed interpolation, and pinning limbs to other limbs.
I aim to make it as simple as possible to actually use. I tried so much animation software that was not only stupidly hard to figure out and use but required you to jump through 50 hoops to use them in your game. Most of the time it didn't work.