~Smiling preferred, though that's not a requirement
~Something transparent or easily made so preferred, though again that's not a requirement (my boredom may take over enough to make a banner out of it)
~Has all or at least most of the body present in the picture
~Doesn't look like something generic (I have the greatest respect for Sugimori but his works are so plain)
~Hopefully by itself in the image, not paired with another Pokemon. If that can't be helped, the other one mustn't get in the way.
~Not in some unusual pose
~...I think you kinda know what I like, right? If you want I can try and think of more details.