Jacko Vance: If you have the copyright for the image, it is still nice to put down in writing that you have it to avoid other people confusing you with stealing art. This is applicable to artist who give you their own work to use, just a short blurb saying "This artist gave me permission to use his work in his profile" etc.
King of Games 1:
#1 If you're using any images you don't own, you would need permission from whoever owns it if you plan on selling for Gaia gold. If it's for free or personal use, then you don't need to credit, although credit is nice to give.
#2 If those entering the lotto need to pay to enter, then a Naruto profile is not OK since you are in effect giving them something that's copywrited in exchange for the gold. If the lotto is free to enter, then this is OK.
Magicookie: Yes you are allowed to use brushes, but please credit them if you can ^_^
Inguz: Eyeballing means copying by eye. If you were to put your own spin into the picture, say add accessories, change the hair, add a detailed background, then this is OK. Having said this, people would pay more for original works of art and it can be frowned upon to have a similar look largely similar to an official artwork.