When Sigil loads your HTML file, it parses it and catches all the image references. These images are then copied to a temporary folder from which they are loaded on demand when they need to be displayed. As you have seen, the images are also renamed and all paths are updated (with a hidden relative path part).
Now, if your image fails to display, then that means something went wrong during the loading process. Image loading is actually now pretty robust, this really shouldn't be happening.
Create an issue on the tracker and attach your HTML file structure (along with the images etc). Follow
these instructions.