Error: this is not an error in fuse as much as it is an error in the tutorial on building the hikr app. The background image that is to be used (https://www.fusetools.com/docs/tutorial/look-and-feel) is in the webp format and not jpg, and if you follow it then you will have errors.
Fix: either explain in the tutorial to use a webp to jpg converter or consider changing the original image back to jpg so that the tutorial steps work