Does Riot accept...

If you mean will they consider anything for implementation that's put here? Hundreds of concepts roll through here regularly; if yours gets noticed and becomes the single most popular concept...the chance is still slim, but better than none.

If you mean is it ok to post any kind of Skin/Champion Concept you have? Absolutely, post away. That's what this board is for.

Don't let negative criticism steer you away from putting it out there; creativity is a gift that's a shame to waste. You never know if Riot will take special interest in it.

I couldn't agree more with Valkyrie.

There are about 10 Skin Concepts or ideas from the Community, that Riot has taken inspiration from. There are also a extremely few amount of Custom Champions used as inspiration. An entire Champion Concept won't be taken. More like 2011 a reaper with a scythe was created by a community member who harvests souls and increases his stats and abilities - 2013 Thresh came.

About skin concepts.. there's a rare case where a guy created a own skin concept on paper and it was Super Galaxy Rumble. Normally Riot doesn't take 1:1 things.