diff --git a/image.go b/image.go
index 9f0a0ed9c..46de3f8aa 100644
--- a/image.go
+++ b/image.go
@@ -230,13 +230,18 @@ func (i *Image) At(x, y int) color.Color {
 //
 // The behavior of any functions for a disposed image is undefined.
 //
+// When the image is disposed, Dipose does nothing.
+//
+// Dipose always return nil as of 1.5.0-alpha.
+//
 // This function is concurrent-safe.
 func (i *Image) Dispose() error {
 	if i.impl.isDisposed() {
 		return nil
 	}
 	theImagesForRestoring.resetPixelsIfDependingOn(i, glContext())
-	return i.impl.Dispose()
+	i.impl.Dispose()
+	return nil
 }
 
 // ReplacePixels replaces the pixels of the image with p.
@@ -245,10 +250,17 @@ func (i *Image) Dispose() error {
 //
 // ReplacePixels may be slow (as for implementation, this calls glTexSubImage2D).
 //
+// When len(p) is not 4 * (width) * (height), ReplacePixels panics.
+//
+// When the image is disposed, ReplacePixels does nothing.
+//
+// ReplacePixels always returns nil as of 1.5.0-alpha.
+//
 // This function is concurrent-safe.
 func (i *Image) ReplacePixels(p []uint8) error {
 	theImagesForRestoring.resetPixelsIfDependingOn(i, glContext())
-	return i.impl.ReplacePixels(p)
+	i.impl.ReplacePixels(p)
+	return nil
 }
 
 // A DrawImageOptions represents options to render an image on an image.
diff --git a/imageimpl.go b/imageimpl.go
index 517526001..c22a75bcb 100644
--- a/imageimpl.go
+++ b/imageimpl.go
@@ -15,7 +15,6 @@
 package ebiten
 
 import (
-	"errors"
 	"fmt"
 	"image"
 	"image/color"
@@ -184,29 +183,26 @@ func (i *imageImpl) restore(context *opengl.Context) error {
 	return nil
 }
 
-func (i *imageImpl) Dispose() error {
+func (i *imageImpl) Dispose() {
 	i.m.Lock()
 	defer i.m.Unlock()
 	if i.restorable == nil {
-		return errors.New("ebiten: image is already disposed")
-	}
-	if err := i.restorable.Dispose(); err != nil {
-		return err
+		return
 	}
+	i.restorable.Dispose()
 	i.restorable = nil
 	runtime.SetFinalizer(i, nil)
-	return nil
 }
 
-func (i *imageImpl) ReplacePixels(p []uint8) error {
+func (i *imageImpl) ReplacePixels(p []uint8) {
 	w, h := i.restorable.Size()
 	if l := 4 * w * h; len(p) != l {
-		return fmt.Errorf("ebiten: p's length must be %d", l)
+		panic(fmt.Sprintf("ebiten: len(p) was %d but must be %d", len(p), l))
 	}
 	i.m.Lock()
 	defer i.m.Unlock()
 	if i.restorable == nil {
-		return errors.New("ebiten: image is already disposed")
+		return
 	}
 	w2, h2 := graphics.NextPowerOf2Int(w), graphics.NextPowerOf2Int(h)
 	pix := make([]uint8, 4*w2*h2)
@@ -214,7 +210,6 @@ func (i *imageImpl) ReplacePixels(p []uint8) error {
 		copy(pix[j*w2*4:], p[j*w*4:(j+1)*w*4])
 	}
 	i.restorable.ReplacePixels(pix)
-	return nil
 }
 
 func (i *imageImpl) isDisposed() bool {
diff --git a/internal/restorable/image.go b/internal/restorable/image.go
index 954e634ea..4eafa1df0 100644
--- a/internal/restorable/image.go
+++ b/internal/restorable/image.go
@@ -261,14 +261,13 @@ func (p *Image) Restore(context *opengl.Context) error {
 	return nil
 }
 
-func (p *Image) Dispose() error {
+func (p *Image) Dispose() {
 	p.image.Dispose()
 	p.image = nil
 	p.basePixels = nil
 	p.baseColor = color.RGBA{}
 	p.drawImageHistory = nil
 	p.stale = false
-	return nil
 }
 
 func (p *Image) IsInvalidated(context *opengl.Context) bool {