Riot Penguin stopped by the thread today to share a few words!
Ahhh! I feel so bad - I don't know how I missed your reply!! Sorry about the absurdly long delay.... :'(
(lovin' the new art btw!)
So regarding in games -- What I do for skins, is when I have a design I like the look of, I just snap a screenshot of the character in game - and then just paint directly on the screenshot -- that way I have all the perspective, proportions, and posing done for me!
This will give you a really good sense of the strengths and weaknesses of the design. In general, you'll find that the head and shoulders are your biggest areas of interest, and you don't really see as much of anything else (so no super decorative belt buckles!).
As far as designing new champions in game -- you can try finding an existing champion with similar proportions to get you started, and then make alterations to better suit the personality of your character. Hope that helps, and sorry again for taking so long to find this thread again!!
(ps, in general if I'm not replying to your replies in a timely manner, you can always just hit me up on twitter, which i check regularly - @riotpenguin)