If you would sue all people who use graphics or sounds without the permission of the creator, maybe half of all webmaster would go to jail. Parts of graphics or photos are re-used in other photos or graphics and if you dig through the large photo sites like getty images or the istockphoto, you will find a lot of design elements, you already saw.
When we were buying our graphics for tradebit, I have found serveral buttons and icons I have seen before and I could swear, they have been taken from there.
For artists there will never be a 100% protection of copyright, but there are some things you can do:
1.) you can watermark your pictures
2.) you publish your content under a creative common license and allow non-commercial use for free. that gives you the credit for your artwork and if someone abuses your creation, it would be easier to prove that it is yours...
3.) keep a record of your publications with date and screendumps of the graphics
Being a digital artist can be tough work sometimes!