Terms of use may change, so you should always check the most up-to-date terms of service. Imgur’s Terms of Service as of December 2016 state that when you upload photos, you grant Imgur a “non-exclusive, royalty-free, perpetual, irrevocable worldwide license (with sublicense and assignment rights) to use, to display online and in any present or future media, to create derivative works of, to allow downloads of, and/or distribute any such file or content.”
In addition, by uploading your photos to Imgur, you agree that people who download your photos may use them for “personal, non-commercial purposes.”