a couple of things. a webcam picture should not be too big to post here. I imagine you are trying to ATTACH it. Go to the How-To Forum and ckeck out the post on uploading to the gallery and the one on posting pictures from the gallery.
If you do need to resize your pictures you can use PAINT its on every windows based computer under accessories. open your picture in paint then select image>stretch/skew. just change the horizontal/vertical percentage to something less than 100 until you get the size you want(make sure they are both the same as not to distorte the image). click ok then save as a new name