I've tried uploading textures with my .obj (and .mtl) file by putting a jpeg inside a folder called "textures", and then zipping the whole thing up. But no textures reach the final render.
I keep reading how one can "manually upload textures in 3D Settings after uploading", but I see no button anywhere to allow this. Nor do I see a pane on the right side, with possible image files to use.
Note I'm not using a 3D program to create these, but the files are coming directly out of the iPhone X. I can display the mesh (with texture) manually, but wanted to see if I could in Sketchfab as well.