Are you scaling down?? Anything more than 120% is going to start breaking up.
We need to know a lot more about what you're doing. It's best to plan your production so that layers are at 100% scale somewhere in the timeline and not more than 120%
Uni student here. Doing a video for a model photoshoot and it seems that whenver I attempt to resize my large images, the image becomes noticeably pixelated.
What does "pixelated" mean? sorry, this is far too vague. Any resizing operation causes resampling no matter whether it's up or down, but specific procedures affect it more than others. Conversely even "scaling" your image in the comp preview to display it smaller in your comp at 50% or whatever res you use will introduce further artifacts, not to speak stuff to do with pixel aspect ratio conversions, hardware accelerated display and what have you... You need to provide a lot more info and screenshots. We don't even know what version of AE you are using on what system. As a start, set your comp res and zoom to 100% when you do your scaling and judge quality this way. Everything else would be pointless. If you still see issues, clarify and provide the necessary details.