Under the directions of many people suggesting this, I installed SDWebImage because I originally had a number of memory warnings. Instead of using NSCache for my images, which helps to populate my collection view from Parse data when my cells are dequeued, I tried SDWebImage's memory cache. What else can I use from SDWebImage to optimize performance other than URL methods? I'm seeing that a lot of their methods involve URLs, but I'm populating from Parse so it seems irrelevant. I've also tried scaling down my images, which helped to a degree, but I still get memory issues. Here's my previous question: iOS Memory Warnings
Given the limited amount of code, it's hard to see what you're doing, however, given you state that URL's are irrelevant I think you're missing an important point.
A PFFile has a property named url. You can be pass the PFFile.url to SDWebImage (setImageWithURL category method) which in turn will load the image and handle the cache for you. You don't need to download the image or do the NSData to UIImage conversion yourself. Should you need to, you can override didReceiveMemoryWarning and instruct SDWebImage to clear its cache as follows;
SDImageCache *imageCache = [SDImageCache sharedImageCache];
[imageCache clearMemory];
Good Luck!