As far as I'm concerned it is not possible to add custom images to Qlik Sense KPI object. KPI uses predefined glyphs. You can watch this here: Qlik Sense - Using the KPI Object (video)
But if you are using Simple KPI from Qlik Branch (Qlik Branch) it seems that it uses icons.eot/icons.woff files placed in themes/default/assets/fonts subfolder. If you want to add your own symbols you'll need to modify extension. You can visit extension page and ask author about this modification (Qlik Branch).