sprite_get_index()

//sprite_get_index(name)
//1000 should be high enough to cover all sprites.
//If not, just increase it :P
var i;
for (i = 0; i < 1000; i += 1)
{
    if (sprite_get_name(i) == argument0)
    {
        return i;
    }
}
return -1;