Update variable.go

Boid/utils/variable/variable.go

 package variable
 
 import (
-	"github.com/hajimehoshi/ebiten"
+	ebiten "github.com/hajimehoshi/ebiten/v2"
 )
 
 var (
 	BirdImage                  *ebiten.Image
+	FishImage1                 *ebiten.Image
+	FishImage2                 *ebiten.Image
+	FishImage3                 *ebiten.Image
+	BackgroundImage            *ebiten.Image
 	WallImage                  *ebiten.Image
 	RepulsionFactorBtwnSpecies float64
 	SeparationPerception       float64