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I took a photo of a painting my mum painted, but i noticed that there was some black in the corners of the photo. I was zoomed in a bit and i had a polarizing filter on at the time. What could have caused it?
http://www.flickr.com/photos/stu213/673 ... hotostream

It is called vignetting. It was most likely caused by the filter on the lens. As the filter extends beyond the end of the lens it is blocking light at the corners. Think about looking through the middle of a toilet roll and how the corners are (significantly) blocked. The same thing is happening with the filter.

According to your EXIF, you were NOT zoomed in. You shot at 18mm.
At this focal length this is to be expected unless you are careful with your filter choice.
Some makers have special narrow filters expressly for wide angle work. They can still cause a problem though especially if you stack them
Compare these two filters. While the one on the left is an excellent filter, I would never use it at 18mm. I would do so with the one on the right though (I have actually used it at 10mm) And if you use a UV filter as a matter of course - many people do - I would remove that before putting the CPL on.


EDIT:
Some things you could do with this photo:
1 Check if your post processing software has a vignette removal tool. Many do. It may help.
2. Add a softer vignette over the top. This will make it look as though it was a deliberate choice.
3. (the simplest) Crop the offending bits off.

If you are using a zoom lens...move back a bit and zoom in a little more. That should fix it. Before I took up photography I thought vignetting was a wine drinking session.