Sounds simple enough. I've done a lot of image resizing before, with PHP. Looking at the suggested "like" websites, looks like you are not requiring much in the way of a front-end. Perhaps just a "here we are" and "here's how to use it" and "here's how to access your pictures or various sizes of your pictures."
As I understand it, never offer picture sizes larger than the original, and in the specified 50 pixel increments up to 250 and in 100 pixel increments up to a max of 3200 square.
The main feature is:
1. the user uploads a picture
2. he can access it by size by offering a "smart" url that includes the sizing information as part of the pathway to the image.
Easy.
Thanks for your time,
-- Phil