Posting competitions can cause spam, and refferal competitions can cause members to use a proxy to create fake accounts. The graphics competition idea sounds create, as it relates to your forum.
A graphics contest is a no brainer. I have a monthly photo contest on one of my forums. Pictures are submitted all month and in the last week the thread is closed and votes are sent to a mod who tallies them up for me. I have two sponsors donating prizes each month.
You can alway use a pol to do the voting but you have to put it together and if you have lots of enteries it's a pain.
Actually that's why people don't participate in them as much.
Graphics competitions are overdone way too much. People tend to dislike the constant graphics competitions, and if your forum has those who don't know anything about graphics, then no one will participate.
Why not have a 'Forum Triathlon' consisting of lets say 3 competitions which can be pretty much anything, though try to mix it up i.e don't have all 3 of them to do with graphics, maybe 1 graphic competition & the others posting/referral etc. The winner can be whoever gains the most points, and points are earned by coming first, second and third place in a competition or just for entering.
I once ran a referral contest on my business forum and in hindsight it was a big mistake. Members paid other members to sign up under their referral ID and these referred members just visited the forum once and made a handful of junk posts. The contest resulted in lots of posts, but they were of such low quality that they did more harm than good to the forum.
You could consider contests like Most Helpful Member, Member of the Week / Month, etc.