There are a series of precautions you need to apply before posting your photos on the web, be it on FlickR, Renderosity or on a personal website.

1. Make sure you use a license restricting any unwanted uses. The 'common license' on FlickR seems way too permissive. I use a Creative Commons license for photos posted on my website that prevents any commercial use or transformation.You also need to make sure that the license terms are understandable and make a direct link to the file whenever it is possible
2. Never, ever post a high res picture on the web. This is begging to have it stolen. There are many possible uses for pictures of more than 700 pixels wide. You can resell it through microstock websites, have them printed, crop them and use them on a website... And there is a fairly large choice of high res pictures posted on the web to do this... including on most of the galleries at Renderosity! I once had a 800 pixels wide photo of a giraffe stolen and used on the French edition of Science magazine. The resolution was enough to make a half page without pixellization.
3. Include a signature on your pictures whenever you post them on the web. It may be cropped out, but may limit some uses of your photos.
4. If you have a personal website, make sure you forbid hotlinking with a script in your .htaccess. Uncontrolled use of your photos can be really damaging, if done on a large scale. I once had 26 of my photos used in an email message sent to thousands of people in the Middle East. Bandwith usage on my website quickly skyrocketed to 8 Gb a day, prompting my ISP to close the website. It took a week to go back to normal.

Fantasy pictures, free 3d models, 3d tutorials and seamless textures on Virtual Lands.