Random image sequence

I'm trying to make a random Image sequence of a word. I have 5 picture boxes, each of them should show a random letter (image of a letter).

The images are in the resources but I don't know how to get the specific images for the specific PictureBox. The images are named like this f1.png, f2.png, ... a1.png, a2.png, ...

If someone could help me that would be awesome. I've seen some C# a few years back, I'm a graphic design student now and I don't know how to program anymore.

Thank you