Do NOT use just any pen!
If you use a pen that is NOT photo-safe, your lovely engagement picture will suffer!
Go to your photographer, local frame shop, craft store or even some of the larger retailers (think Target) and look in their craft section for a photo-safe pen.
I have two quick stories to share:
1) a friend of mine used this same idea. One of the guests had a drink or two too many and wrote some LARGE and profane items on their beautiful photo matte. No way they were displaying that in thier home!
2) My husband and I did it too, but didn't have any problems! We didn't serve alcohol (for religious reasons) at our reception!
My point: If you do this, think very carefully about your guest list. If there is someone there you think has the potential to ruin this for you - skip the idea.
But trust me on the pen - go get a photo-safe one.
Best of luck!