Store multiple sizes of an image or just store the main and resize?

Question

I'm working on a CMS and I'm trying to figure out the common practice for doing a REST style image request. I have three sizes, small, medium, and full. My thought is to store only the full and write a function that will resize on each page request. This has obvious cpu costs. The other end is that I could store all three sizes and only calculate on upload, this seems to waste space.

My environment is an intranet, so relatively low requests and a high number of images stored. Thoughts?

Note: I realize I don't have to really worry too much since it's intranet and either solution will work, just wondering which would be preferred for knowledge sake.

Answer 1

Another option is to maintain a cache of resized images. Serve up the ones that are available. Create new ones if they are not available. Delete images that have not been requested for a while.

This will be a compromise between the CPU and storage issues.