I've gently and kindly said no on some occasions. For reasons most of you already covered. Running late, busy getting to a shoot or event I'm participating in, food or heavily inconvenienced at the time (like holding everyone's stuff while they get food or use the restroom). I had once declined a photo while in the bathroom---but mentioned if you give me a few minutes I can be out and you can get your pic then. XP
I always feel bad declining a photo and am very apologetic.
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Or here Adnarimifcation Cosplay
Next Convention: Katsucon
Seth Nightlord - Trinity Blood
Death Maiden - Diablo III
Wanderer - Journey