Storing images for photo album?

I'm writing a photo album in PHP. The idea is to convert user-uploaded images to PNGs with unique IDs (1.png, 5.png, etc.), and cache them at certain sizes as needed. Now I'm running into an out-of-memory converting user images to PNGs, and I'm not sure what to do: copy them in whatever format the user used and convert to JPEGs for output? Use some other format for storage? -- The above message is encrypted with double rot13 encoding. Any unauthorized attempt to decrypt it will be prosecuted to the full extent of the law.