One pet peeve I have that deals with fan "art" is -what seems to be- a majority of cosplayers. There are two aspects to the peevishness.

1- Lack of Real Effort Given to the Costume.
A: If you are going to dress up as a character... then do just that. Don't put on the suit/dress of the character, but don't use a wig to match the character's hair. I've seen people say "I'm cosplaying as Zoro!" ...but still have brown hair. Or worse, they DO have a wig but its actually blue instead of green. W-T-F! Another part of this is try to make a costume look like the character does, only to drastically change something of it that causes a WTF moment. I once saw a Izo (One Piece- Whitebeard Pirates 16th commander) cosplayer try to look like his outfit ...only for his costume to end at knee level! Izo's 'dress' was full length.

B: Another part of this say you are one character but wear something the character wouldn't wear. Usopp of One Piece would not wear a 3 piece suit. Ciel of Black Butler wouldn't wear a maid costume. XD

C: Costumes can be cheap ...but they do not have to LOOK like they are cheap! Same  Izo costume mentioned above had made the collar of the outfit oversized and looked as if it was foam filled... you know like the Retail store $20 or less (generalization there)Halloween costumes that make the collars stick out with foam. It really did make the Izo costume look cheap, like "it could have came off the rack at Wal-Mart" cheap.

2. Oversaturation of Pictures.
A: Why is it that many Cosplayers must post a dozen pictures of the same session/costume? (Events will be covered in 2.B) I get so tired of looking at some moron post 5 (sometimes 8![[or more!]]) pictures and the only difference is a slight change of posture.
I swear its like "oh I moved 3 degrees to the left! Take a picture!"
"I tilted my head to the right! Picture!"
"I'm holding a doll of my character! Picture!"
"I'm now tilted my head down to put my cheek on the doll's head! Picture!"
"I'm smiling! Picture!" someone tell her she/he already did that and she/he answers "No! I was grinning, there is a difference! I'm actually doing the character's way of smiling! Now take the DAMN picture!"

B: Events are a 50/50 match here. While an event has the potential to have different scenes, many posters do not post different scenes. Many post several pics of same scene with same people. Like the spammer above, the only change in pics is slight change of stance.

A note to Cosplayers=
Please post the best picture (or best 2 pics if they differ enough) and do NOT post  multiple pics of the exact same thing that vary only slightly change like the posing. Make a blog or post at sites like photobucket for the entire collection for that session.
My peeve with this is that I try to look for art for some manga character, and I have to sludge through a slew of cosplay pictures. The multiple pics just annoy many, but you post the best one, its more likely to be positively received. I look at cosplay pics, but if there a 5-10 of the same person-character, I WILL skip over it.