That's your cache folder. The image should be in a subfolder with a name like blob5.png.
Do you see a png file created in ProcessMonitor? If you view the report in launchpad, does the image stay in the cache or does it get created and deleted right away. What we need to figure out is why the image is not being persisted on disk.
It could be your cache settings on the Webi Processing server. From my tests, if the path to the image uses something like TMP*1*2, then the cache is not enabled. With cache enabled, the path to the image uses a path like dxXMLdraft.drilloff and the image is in a subfolder. If your cache settings are like the ones I mentioned above, then maybe your timeout is set to zero. Getting the image to stay in the cache directory is the key to getting the image to display on your REST html export.