Add animated GIF support

Note that this is for GIFs only (and not Webp).

Fixes #5030

resources/image.go

@@ -19,6 +19,7 @@ import (
 	"image"
 	"image/color"
 	"image/draw"
+	"image/gif"
 	_ "image/gif"
 	_ "image/png"
 	"io"
@@ -346,6 +347,15 @@ func (i *imageResource) decodeImageConfig(action, spec string) (images.ImageConf
 	return conf, nil
 }
 
+type giphy struct {
+	image.Image
+	gif *gif.GIF
+}
+
+func (g *giphy) GIF() *gif.GIF {
+	return g.gif
+}
+
 // DecodeImage decodes the image source into an Image.
 // This an internal method and may change.
 func (i *imageResource) DecodeImage() (image.Image, error) {
@@ -354,6 +364,14 @@ func (i *imageResource) DecodeImage() (image.Image, error) {
 		return nil, fmt.Errorf("failed to open image for decode: %w", err)
 	}
 	defer f.Close()
+
+	if i.Format == images.GIF {
+		g, err := gif.DecodeAll(f)
+		if err != nil {
+			return nil, fmt.Errorf("failed to decode gif: %w", err)
+		}
+		return &giphy{gif: g, Image: g.Image[0]}, nil
+	}
 	img, _, err := image.Decode(f)
 	return img, err
 }