Please check the script reference.
Resources.LoadAssetAtPath is an editor only function, you can not use it in builds.
Resources for builds must be within a Resources folder and are then loaded by name relative to this resources folder (or just by name if you know its surely unique), in both cases through Resources.Load
On top of that you should call resources load in the start function and store the image into an image handle. Otherwise you will kill the game more sooner than later as you are currently reloading the image several times per frame without ever clearing the previous one