Well, to be perfectly accurate, it's:
TerrainArt/LordaeronSummer/Lords_Grass.blp
For a custom texture to overwrite the grass in the Lordaeron tileset, double-click the desired texture file in the Import Manager and replace the text with the bolded text above that I posted. You will need to save the map afterwards and reopen the World Editor for it to be viewed in the editor.
Hope this helps!
~Craka_J