diff --git a/data.go b/data.go
index 6eae282..2e2c226 100644
--- a/data.go
+++ b/data.go
@@ -28,6 +28,8 @@ const (
 	optFit             = "fit"
 	optFlipVertical    = "fv"
 	optFlipHorizontal  = "fh"
+	optFormatJPEG      = "jpeg"
+	optFormatPNG       = "png"
 	optRotatePrefix    = "r"
 	optQualityPrefix   = "q"
 	optSignaturePrefix = "s"
@@ -71,6 +73,9 @@ type Options struct {
 	// Allow image to scale beyond its original dimensions.  This value
 	// will always be overwritten by the value of Proxy.ScaleUp.
 	ScaleUp bool
+
+	// Desired image format. Valid values are "jpeg", "png".
+	Format string
 }
 
 func (o Options) String() string {
@@ -97,14 +102,18 @@ func (o Options) String() string {
 	if o.ScaleUp {
 		fmt.Fprintf(buf, ",%s", optScaleUp)
 	}
+	if o.Format != "" {
+		fmt.Fprintf(buf, ",%s", o.Format)
+	}
 	return buf.String()
 }
 
 // transform returns whether o includes transformation options.  Some fields
 // are not transform related at all (like Signature), and others only apply in
-// the presence of other fields (like Fit and Quality).
+// the presence of other fields (like Fit and Quality).  A non-empty Format
+// value is assumed to involve a transformation.
 func (o Options) transform() bool {
-	return o.Width != 0 || o.Height != 0 || o.Rotate != 0 || o.FlipHorizontal || o.FlipVertical
+	return o.Width != 0 || o.Height != 0 || o.Rotate != 0 || o.FlipHorizontal || o.FlipVertical || o.Format != ""
 }
 
 // ParseOptions parses str as a list of comma separated transformation options.
@@ -153,6 +162,11 @@ func (o Options) transform() bool {
 // The "q{qualityPercentage}" option can be used to specify the quality of the
 // output file (JPEG only). If not specified, the default value of "95" is used.
 //
+// Format
+//
+// The "jpeg" and "png" options can be used to specify the desired image format
+// of the proxied image.
+//
 // Signature
 //
 // The "s{signature}" option specifies an optional base64 encoded HMAC used to
@@ -174,6 +188,7 @@ func (o Options) transform() bool {
 // 	100,r90   - 100 pixels square, rotated 90 degrees
 // 	100,fv,fh - 100 pixels square, flipped horizontal and vertical
 // 	200x,q80  - 200 pixels wide, proportional height, 80% quality
+// 	200x,png  - 200 pixels wide, converted to PNG format
 func ParseOptions(str string) Options {
 	var options Options
 
@@ -189,6 +204,8 @@ func ParseOptions(str string) Options {
 			options.FlipHorizontal = true
 		case opt == optScaleUp: // this option is intentionally not documented above
 			options.ScaleUp = true
+		case opt == optFormatJPEG, opt == optFormatPNG:
+			options.Format = opt
 		case strings.HasPrefix(opt, optRotatePrefix):
 			value := strings.TrimPrefix(opt, optRotatePrefix)
 			options.Rotate, _ = strconv.Atoi(value)
diff --git a/data_test.go b/data_test.go
index 2f19ef2..5f79321 100644
--- a/data_test.go
+++ b/data_test.go
@@ -31,12 +31,12 @@ func TestOptions_String(t *testing.T) {
 			"0x0",
 		},
 		{
-			Options{1, 2, true, 90, true, true, 80, "", false},
+			Options{1, 2, true, 90, true, true, 80, "", false, ""},
 			"1x2,fit,r90,fv,fh,q80",
 		},
 		{
-			Options{0.15, 1.3, false, 45, false, false, 95, "c0ffee", false},
-			"0.15x1.3,r45,q95,sc0ffee",
+			Options{0.15, 1.3, false, 45, false, false, 95, "c0ffee", false, "png"},
+			"0.15x1.3,r45,q95,sc0ffee,png",
 		},
 	}
 
@@ -72,6 +72,7 @@ func TestParseOptions(t *testing.T) {
 		{"r90", Options{Rotate: 90}},
 		{"fv", Options{FlipVertical: true}},
 		{"fh", Options{FlipHorizontal: true}},
+		{"jpeg", Options{Format: "jpeg"}},
 
 		// duplicate flags (last one wins)
 		{"1x2,3x4", Options{Width: 3, Height: 4}},
@@ -79,13 +80,14 @@ func TestParseOptions(t *testing.T) {
 		{"1x2,0x3", Options{Width: 0, Height: 3}},
 		{"1x,x2", Options{Width: 1, Height: 2}},
 		{"r90,r270", Options{Rotate: 270}},
+		{"jpeg,png", Options{Format: "png"}},
 
 		// mix of valid and invalid flags
 		{"FOO,1,BAR,r90,BAZ", Options{Width: 1, Height: 1, Rotate: 90}},
 
 		// all flags, in different orders
-		{"q70,1x2,fit,r90,fv,fh,sc0ffee", Options{1, 2, true, 90, true, true, 70, "c0ffee", false}},
-		{"r90,fh,sc0ffee,q90,1x2,fv,fit", Options{1, 2, true, 90, true, true, 90, "c0ffee", false}},
+		{"q70,1x2,fit,r90,fv,fh,sc0ffee,png", Options{1, 2, true, 90, true, true, 70, "c0ffee", false, "png"}},
+		{"r90,fh,sc0ffee,png,q90,1x2,fv,fit", Options{1, 2, true, 90, true, true, 90, "c0ffee", false, "png"}},
 	}
 
 	for _, tt := range tests {
diff --git a/imageproxy.go b/imageproxy.go
index 4afc67f..5c0db49 100644
--- a/imageproxy.go
+++ b/imageproxy.go
@@ -306,7 +306,10 @@ func (t *TransformingTransport) RoundTrip(req *http.Request) (*http.Response, er
 	// replay response with transformed image and updated content length
 	buf := new(bytes.Buffer)
 	fmt.Fprintf(buf, "%s %s\n", resp.Proto, resp.Status)
-	resp.Header.WriteSubset(buf, map[string]bool{"Content-Length": true})
+	resp.Header.WriteSubset(buf, map[string]bool{
+		"Content-Length": true,
+		"Content-Type":   opt.Format != "",
+	})
 	fmt.Fprintf(buf, "Content-Length: %d\n\n", len(img))
 	buf.Write(img)
 
diff --git a/transform.go b/transform.go
index 4536c61..4c91913 100644
--- a/transform.go
+++ b/transform.go
@@ -16,6 +16,7 @@ package imageproxy
 
 import (
 	"bytes"
+	"fmt"
 	"image"
 	_ "image/gif" // register gif format
 	"image/jpeg"
@@ -46,6 +47,10 @@ func Transform(img []byte, opt Options) ([]byte, error) {
 		return nil, err
 	}
 
+	if opt.Format != "" {
+		format = opt.Format
+	}
+
 	// transform and encode image
 	buf := new(bytes.Buffer)
 	switch format {
@@ -74,6 +79,8 @@ func Transform(img []byte, opt Options) ([]byte, error) {
 		if err != nil {
 			return nil, err
 		}
+	default:
+		return nil, fmt.Errorf("unsupported format: %v", format)
 	}
 
 	return buf.Bytes(), nil