For those who might want to use Visio to frame a picture with a custom mat, here's a smart shape to help you out!
Either upon placement or upon double clicking, the shape will request both the outside dimensions and the inside opening dimensions. The shape itself has two regions of selection. The main shape which will bring up a right click context menu that will allow you to select single or double mat options, with either none, single, or double lines. The other option allows you to lock the aspect ratio the the middle line / inner mat fill shape. The inner line color and thickness is also part of the outer shape selection. The inner region may be selected to change the color of the inner fill, the color of the intermediate (middle) line, and the line thickness. Also, for convenience, you can enable / disable locking the aspect ratio of the middle line shape.
Note: this shape does not include any framing shapes, just the mat.
Enjoy!
Wapperdude
Update: modified the shape to remove some of the protections. Should be easier to use.