I am clicking on the little square box that is "Insert Image Link"
If you do it that way, you need to right click the image you want to share, select "copy image address", and then when you click the "insert image link" button you will need to right click and "paste" the image address in. You may also have an option for sharing the direct image address from Photobucket, as I think I recall seeing that in Flickr as well. Just keep in mind that "Insert Image Link" button needs the direct web address to the image you're sharing.
These are a couple helpful shortcuts to remember when copying and pasting in Windows
CTRL+X = CUT
CTRL+C = COPY
CTRL+V = PASTE