I get tired of always having to go open an image in the UV/Image Editor before being able to choose it from the Stencil Image drop down in the Slots Mask option.
I checked the python source in the UV Image Editor for the Open Image button, and copied that to the place in the Stencil Image source python. Now I saved it and can open a picture as a Mask Stencil directly there instead of the trip to the other editor. Can this be a simple fix in source then that will be usable out of the box for new users? I feel like there might be a few others that have this issue in places in Blender.
The line is the 1281 mark where I added the , open="image.open" to the line there, and that gave me the image open button.