Do not automatically assume that your copyright has been infringed.
Under the copyright laws of the UK and most other countries, works protected under copyright law can be ‘incidentally’ featured in another artwork, sound recording, film or broadcast, without seeking the prior permission of the copyright owner. ‘Incidental use’ usually means that the work is in the background of another image, or permanently sited in a public place, and not the main feature of the artwork, recording or broadcast. This exemption is not automatic, and the exact use of the image must be considered.
Other exemptions include ‘fair dealing’ and ‘substantial part’ exemptions, discussed elsewhere.
If you think that the use of your work is still unauthorised, and falls outside this exemption, then the only way to assert your copyright is through legal action. Copyright cases taken to court are generally extremely costly, and breach is very difficult to prove – consider the financial repercussions carefully before embarking on this course of action.
Also, remember that there is no copyright in an idea, only in the manner of its expression. It will therefore not be an infringement of copyright to use your underlying ideas in a new work of art.