How to crop multiple pictures for a time-lapse so they fit over each other perfectly?

Yesterday I slipped with my bicycle and scraped my knee pretty badly. To make the best out of it, I decided to take a picture of it every 6 hours and make it into a time-lapse kind of slideshow to get a closer look on the healing process. I’ll probably do it for around a week, which would make about 30 pictures.

I set up a tripod and chair to keep the perspective the same, but of course I can’t position my knee perfectly every single time, so I’ll have to slightly crop the pictures afterwards so they all line up with each other perfectly.

How could I do that? What software/workflow is a quick way to exactly crop my pictures? I assume using something like a semi-transparent layer in Photoshop to put one image over each of the others could work, but it would be pretty annoying to do 30 times in a row. Anyone knows a more smart/economic solution?