Photo copyrights are a very hard thing to try to prove legally. A lot of factors go into determining if you truly indeed hold the copyright to an image.
By far the largest thing they look at is if you do that professionally and have a body of work to show is yours. This does not mean you have a bunch of pictures on your computer you can show them. It means having imbedded data in your images that detail who owns the image and signed releases by previous customers. Copyrights are not explicit nor implied when one snaps a picture. A non-professional can copyright images as long as information exists that can prove the image is thiers.
Where the image is shot can also determine whether you can even hold copyright to the image. Public places such as zoos, malls are fine... A private fish store would be very questionable unless you were there specifically to take pictures of thier stuff.
Now taken that this picture was posted on a public board and did not have anything visually or digitally to signifiy that it was a copyrighted image the only thing you can hope for is they cave when asked to remove or compensate you for your image.
I am a professional photographer and I belong to a professional organization that helps in copyright cases. Even with that organization backing me and tons of contracts and work for over 3 years I still would have a hard time getting anything from a person or small organization for stealing my work. I can become a nuisance
![Smile :) :)](https://cdn.jsdelivr.net/joypixels/assets/8.0/png/unicode/64/1f642.png)
Large companies are the only ones I could get recompense from but they are usually very careful about what they use.
To the person that said it was ok to download copyright photos to your local machine and use in screen savers and such I would like to let you know that a copyright is a copyright and as such unless you are given explicit rights to use that image for that purpose you are in violation of copyright laws. Images that have copyrights can't be scanned, copied, displayed (outside your home) or manipulated without getting the express consent of the copyright holder to do exactly what you wanted to do to an image.
I hope this helps in the least bit