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.”
photo copyright law 
I don’t think it is legal, First you don’t need to login, sign up or sign any terms of service when uploading or acknowledge anything, They can’t have a license to pirate material on the website. You just load the website and drag and drop upload or click the button. Where is this so called terms of service box agreement at? Also they would be claiming license to illegal content they may end up on the website.