I had made a recent wallpaper using a picture from Sky Lounge Garden (it isn't anything adult). But I wasn't sure if SLG's art modifications allowed here.

Just add the the render isn't yours (and since you know the source you might as well give them their props) to be on the safe side and you should be alright.

Just read the FAQ and the Minitokyo Policy in the support section and you should be just fine.