Many people talk about getting that perfect white background in their photos. I have already had two entries on how to do this with your camera settings. There is another way to tweak them as well just in case they didnt come out as white as you were hoping for.
Many people have post processing programs like Photoshop and the likes. If you dont have one of these programs though there is a free alternative. Gimp is a free program that does a lot of what photoshop can do http://www.gimp.org/ is the site.
Here is a quick reference on a quick fix. Open Gimp then open your photo with the not so white background. Then go to Colors>levels
Drag your cursor to the white eyedropper and click
Then pick what should be the whitest spot in your photo. I am picking the highlight reflected on the surface from one of the beads. You can see an arrow pointing to where I am talking about. Then click on it with the eyedropper. Click ok to save the changes.
Here are the results.
Full size before and afters
Depending on how off your original is your picture may require more tweaking. We can get into that another time.