A recent commit to blender trunk broke the yafaray exporter, the rendered image is not displayed correctly in the blender image editor, and there seems to be at least one python error associated with this problem.
The blender commit that caused the problems:
Just guessing, but it seems some API changes altered the method for sending tiles to the image editor and canceling the render, and maybe some other stuff as well.
Here's the specifics on the python error mentioned above:
yaf_export.py", line 350, in render self.end_result(result)
TypeError: YAFA_RENDER.end_result(): required parameter "cancel" not specified
The yafaray exporter still works perfectly with blender revisions r50383 and earlier.