Try to set region:
self.view.add_regions('swi_breakpoint_active', self.lines(enabled), get_setting('breakpoint_scope'), icon=breakpoint_active_icon, flags=sublime.HIDDEN)
in console:
Unable to decode /Users/stanislav/Library/Application Support/Sublime Text 3/Packages/Web Inspector/icons/breakpoint_inactive.png
I have tried png, jpg, gif, relative and absolute paths.
in ST2 all works fine, maybe something wrong with path? or some specific icon format?