When I was little and my parents took us to art galleries (which they did a lot, and now I take my kids) we were always allowed to choose one postcard from the gallery shop. I still love to do that, though I buy more than one these days :-)
I don't know what happened to that old postcard collection. I'd love to look through it again. Lost in a move I expect.
Here is one choice that I remember, it is Ophelia by John Everett Millais.
It came to mind the other day and I went looking online for it. I still love it. My taste mustn't have changed since I was ten!