I don't see anything wrong with artist's voting for their own work. In the past, I've voted for my own work if I thought it was the best -- which is most of the time, TBH. But, sometimes I've voted for other artists' work, when I thought they were better. For example, in the design competition for this pokemon, I didn't vote for my Jack-O-Lantern design. I voted for the "lava snake", I think.
The only "sportsmanship" issue I have, is when artists bash or criticize other artists' work during the poll. I've ranted vociferously about this in the past, so I won't do it again here. I think Hazmat gave the best analogy for these art polls -- all artists should imagine that they are in an art gallery standing in front of their art, with all the other artists and artworks nearby. Feel free to comment on your piece, but don't comment on other artwork, unless it is a compliment.