Like you say, the copies on the internet are of poor quality, and that's usually so people can't print them out for themselves. Unlike literature, art, like music, is owned by publishing groups and by interested parties, ie. the gallery or private owner of the said piece of art even after death of the artist. So if you want a copy, you'll have to ask them for permission if you want a decent print. Most galleries do prints of the paintings they own, but only tend to go for the more popular images. Some can make prints of specific paintings, but they're usually pricey.
The Tree Of Life is in the Osterreichisches Museum fur Angewandte Kunst in Vienna, so you'll have to go through them.