Not the same issue but I ended up here because every one of my images was showing an orange 'missing' box. The images had been moved at some point, so the base images folder was now wrong. It took me forever to figure out how to fix it, so here it is:
To fix missing images (Spine 3.8), you must be in SETUP mode (click ANIMATE at the top left so it says SETUP), then in the Tree panel under Hierarchy, under your top object, at the very bottom look for an Images folder. It should appear under Skins and Animations but is hidden unless you're in SETUP mode (whyyyy). Click it, and in the Image Files panel below, change the Path to the root of where your images are located. Eg "./Images" or just "." for the project path.
Edit: yeah I definitely mean 3.cool smiley face, not 3.8 😃