I've bought a gorgeous heavy full size leaning mirror...

[Behooven]

how do I lean it against the wall without marking the paint though?

You could get some of those soft sticky pads, the type you stick under whatever to stop them scratching surfaces, and put some strips along the top edge at the back?

Good idea!

Please ensure it is safely fixed if you have young children.

www.huffingtonpost.com/2013/06/08/boy-killed-by-mirror-falling-hugo-boss-store_n_3409439.html