Sounds like nothing more than a couple of keyframes with the anchor point exactly at the center of the image. You can get it there by turning on snaps.
You can also just keyframe one image rotating and then add an expression with a minus sign in the front of it using the pickwhip. Just select the image 1 and image 2, press r to reveal the rotation property for both layers, select only the rotation property in Image 2 while holding down the Alt/Option key, type - then grab the pickwhip and drag it to the rotation property of Image 2. You should get an expression that looks something like this:
- thisComp.layer("Image 2").transform.rotation
Now all you have to do is create a mask on one or both of the images to reveal only half of the circle.
This is real basic AE stuff. Sounds like you are new. I would start on the Adobe After Effects CC product page and follow the link that says New to After Effects? Get the basics then check out the beginner section.
New!? I've been doing this a whole 30 minutes! Thank you for pointing me in the right direction. Now to see what kind of damage I can do!
Follow the link and get some basics before you do anything else. It will save you a lot of time. AE has a billion combination of switches and I've never met anybody that could figure it out without reading the manual.