I had some success creating a PictureFillSymbol and resizing it when the map changes, but it does not look that great when the map is zooming, and it is not 100% reliable. It would be nice if there was a 'stretch' option on the PictureFillSymbol.
Is there a better way that I could do this?
This is disappointing. In previous versions we were able to Create a FillSymbol and fill it with an ImageBrush and it would fill the symbol. In this version, the PictureFillSymbol tiles rather than stretches to fill. This seems to me more like a bug than a missing feature.
I assume by "previous versions" you are referring to the "WPF SDK", which is a different product. The XAML based symbol were THE biggest performance problem and it wasn't a sharable symbol type, printable or in general fit with the entire ArcGIS system. As mentioned we're working on adding direct read of rasters, which is the "real" way to display georeferenced images in a performant way that can also support high-res images - let's be honest: the symbol approach in WPF to do georeferencing was a bit of a hack