Depends on how much you want to spend. You can download a photo from here print it on decent paper and frame it. Or if you're looking for something a bit more regal, crown-and-sceptre-type thing, try an antique shop; you'd be amazed what you can find if you look carefully.
Seem to remember messes being entitled to pictures of Liz and Phil through the system depending of the size of the mess in question. Eg a very large mess membership can warrant a large colour print signed personnaly by HM whereas a smaller mess might only get a copied signature. The G4 chain might be a place to look and save you money.
The National Portrait Gallery has a service where they will do a quality print of any of their pictures on paper of your choice (photographic, canvas etc etc). You can do it through their website. Worth a look.
There must be a few kicking about from all those messes that folded up over the years, I know of one senior jock who has a lot of pictures of HRH on his walls.
Mind you most folk in the UK carry a picture in their wallet.