Create a polygon filled with a tiled image in c#

I'm creating an application which visualises a picture frame as the user designs it. To create the frame I am drawing 4 polygons which represent the physical bits of wood and using a TextureBrush to fill it.

This works perfectly well for the left and top edges. However, for the bottom and right edges this method isn't working. It appears to me that the TextureBrush is tiling from the point (0,0) on the image and not within the polygon I've drawn. As a result, the tile doesn't line up with the polygon. By adjusting the size of the image I can get the tile to line up perfectly.

How do I create an arbitrarily positioned polygon and fill it with a tiled image, starting from the point (0,0) within the polygon, not the canvas?

I'm not attached to FillPolygon and TextureBrush if there is a better solution.

Example