Where is the aneurysm in this picture?
Your image comes from the wiki page Aneurysm. The figure legend on the wiki page identifies it as:


  Angiography of an aneurysm in a cerebral artery


I am not an MD so I chose to first dig up a similar appearing cerebral angiography with the aneurysm identified (dark protruding spot indicated by the arrow):

Source: WestJEM, UC Irvine 

Taking into account the normal vasculature as seen on this graph from UC Irvine, and given the similarity between the identified aneurysm in this angiopgraph and your wiki angiograph, I tentatively have identified the red-encircled structure in your original image as an aneurysm: