I would load to GRP4 since that's where the default sprite sheet is and you want to "overwrite" it. Background tile is GRP5 btw.
I'll show you an example using my game. Let's say I want to put a strawberry and the first picture of ninja.
'Strawberry
LOAD "GRP4:_X"
SPSET 0,1
SPOFS 0,100,120
'Ninja
SPSET 1,80,64,64,64,1
SPOFS 1,120,180
Explanation next post, or see HELP
1そうだね 未プレイ