I didn't see a tutorial section anywhere, so I'm posting here. This simply allows you to click a sprite and save it as a file. This isn't perfect - it doesn't include a menu (like in Windows), and it may capture gray area aroudn the sprite (or background) depending on how the sprite is shaped. Either way, this is how you do it.
temp_id = instance_position(mouse_x,mouse_y,all);
if(temp_id != noone) {
var tx, ty, tsw, tsh;
tx = temp_id.x;
ty = temp_id.y;
tsw = sprite_get_width(temp_id.sprite_index);
tsh = sprite_get_height(temp_id.sprite_index);
screen_save_part("sprite.bmp",tx,ty,tsw,tsh);
}
That can go in global left pressed.