Hey Tim, if you add the file to Sketchfab as a base color, one reason you might not see it is if it is too glossy. In the 3D settings, change the Shading setting from Lit to Shadeless.
Another thing that might help is watching this course. The last lesson describes the relationship between models and textures, and how to work with them between different programs.
https://cgcookie.com/course/understanding-blender-data
