Instructions for Blender 2.82
Listen me and i solve your problem. When you think you have inserted the right textures to your object.
File/Export/Collada DAE
In the export window select the right-top gear button and in Texture options select the voices "Only selected UV Map" and "Copy". Export the DAE file. After that, export UV Map Texture and open Second Life.
In Second life import dae (Deselect checkbox "Include Texture" in Loading Option) after this import image (10LS). Spawn your Object in the Scene and change Texture Option.