Tutorial on real leaf canvas art: press leaves overnight (wax paper & books), paint the base of canvas as you like it, apply ModPoge (they used metallic MP) to seal leaves. Treat with glazes or paint as desired. Adhere to canvas.
Elmer's Glue on Canvas, then paint the whole thing. I have totally done this, but with a hot glue gun. Elmer's Glue seems to be a better idea since hot glue leaves behind strings and is harder to work with.