An easel looks really nice, but it becomes difficult for guests to write at that angle. And some pens won't write at that angle either. I'd recommend laying it on a table and having a bunch of pens around it. You could always place a
couple of books or something under the top portion so it's slightly upright and nicer-looking.
I do have one recommendation that often goes overlooked. (I used to be a picture framer and I saw this all the time) Don't display the mat all by itself, display it in a frame (with no glass obviously). The lip of a frame will cover about 1/4"
of the edges of the mat. 99% of guests don't think about this when they're looking for a place to sign and will sign right to the edge. And, when you frame it, some of the writing will be covered up. The best way to prevent this is to display the mat
within a frame. Even if you haven't chosen a frame yet, consider getting a cheap temporary one just for this purpose.
As far as what photo to display, I've seen a variety of photos... an engagement photo, a prom photo, baby photos side by side, or just a random photo of the couple. A friend of mine, knowing that they had planned to display a wedding photos
after the wedding, displayed a goofy photo of herself and her fiance.
Good luck!