If you want a logical answer...
The way the contest feature is designed by FAA... The voters have several alternatives to cast their votes. One way is to go through every selection one by one on the vote page (where each page shows only one image entry). Sometimes there could be several hundred pages or even more than a thousand entries. (This is one reason why there are many complaints about the "contests" that allow 10 entries for each member and why some members do not enter those said contests).
There is another way to vote, by going to the Leaderboard page however, the image must already have a vote before it can appear there. No surprise, allegedly most votes are cast on the leaderboard page.
Assuming the leaderboard page which keeps a running tally of votes per image has by default narrowed down the allegedly best images by previous votes (both from the voting page and/or the Leaderboard page), this could in theory, sort out the best (or favorite, or most popular, or most solicited) of the contest's images to choose from, on a much shorter list, and at a comparative glance.
Problem...
Since assuming, most people vote from the Leaderboard page, and since an image must have at least one vote just to appear on the Leaderboard... and one cannot depend on an image having received a vote... the best solution is to vote for your own image to assure it appears on the Leaderboard.
Aside from the logic, why would there be anything wrong with voting for your own image?
Sidebar... I happen to immensely dislike those contests where the host dictates that members are not allowed to vote for their own image. I for one am greatly upset when anyone tries to tell me how to vote in a contest. If an image is eligible for voting on, and the members are eligible for voting, where is there a dependency?
Yeah yeah, we all know (except for one stubborn contest host I'm aware of) that these so-called "contests" aren't really contests at all, but just a means of getting greater exposure for your work (thank you for that). Assuming this is true, then why not call them something other than a "Contest"? Also, if the contests are meant for FAA artists to gain additional exposure for their work, why do so many contest hosts, make such long lists of demanding criteria for the contest submissions? They should instead make it as easy as possible for entries to be made, not more difficult. (just my 2 cents).
A possible exception...
If these were REAL, unsolicited contests, and not just to gain additional exposure, I might say that voting for yourself IF YOU ARE ALSO THE HOST of the contest, might be an advantage for you thus unethical. Since we're told the contest premise is only for additional exposure and now, FAA actually encourages solicitation for that exposure, then I'm not concerned with the ethics of voting for my own image if I'm the admin host.