Print website stole my image.

You are in the UK, infringer is in Greece. Use the EU Small Claims procedure for copyright infringement through unauthorized use ( https://e-justice.europa.eu/content_small_claims-42-en.do ). Before that, send the infringer a letter requesting a copy of their license for use. Include in that an offer to settle should a valid license not exist. Set a deadline and reasonable amounts (based on the time your content was used and what you would have charged for the use). Save all communication (if they ignore you, make a note that there was no response). Once the deadline has passed, file your claim for damages and include all the material you have collected. There is a PDF at the link above, see that for the how-to and what you need to include. If you Google (copyright, small claims, eu) you'll no doubt find better information. As you're in the UK, a call to the IPO would provide you with more information ( https://www.gov.uk/government/organisations/intellectual-property-office ).

To have the image removed, skip asking the infringer. Document the infringement thoroughly (everything you could reasonably need to prove the infringement occurred as you claim) and then contact the service hosting the images. If the host is a US one, use a DMCA notice. If in the EU, use their contact information to notify them of copyright infringement (look under contact/abuse on the host website). Use one of the many form letters to be found online for that and be sure to include everything required depending on where the host is.

Are the prints being sold by the service itself (without user accounts) or has someone created an account on the service and (more likely than not) broken the ToS agreement by uploading other people's content? If the latter, report the account to the service and have the service take action against the account as well (after your deadline for settlement has passed - on the off chance they pay for the time used up until the deadline set, anything past that would require the purchase of a new license, rather than a retroactive one for use up to that point).

/r/photography Thread