I have a patch for libgd and jpeglib7 to allow exactly that. However a manual scale is still necessary if you like to have a good result.

here is a suggested patch (against 5.3). i have made this patch so that it uses this prescale value only against bundled libgd. let me know , if this makes sense to you. 

Index: ext/gd/libgd/gd_jpeg.c
===================================================================
--- ext/gd/libgd/gd_jpeg.c	(revision 289063)
+++ ext/gd/libgd/gd_jpeg.c	(working copy)
@@ -260,21 +260,21 @@
 	gdFree (row);
 }
 
-gdImagePtr gdImageCreateFromJpeg (FILE * inFile, int ignore_warning)
+gdImagePtr gdImageCreateFromJpeg (FILE * inFile, int ignore_warning, int prescale)
 {
 	gdImagePtr im;
 	gdIOCtx *in = gdNewFileCtx(inFile);
-	im = gdImageCreateFromJpegCtx(in, ignore_warning);
+	im = gdImageCreateFromJpegCtx(in, ignore_warning, prescale);
 	in->gd_free (in);
 
 	return im;
 }
 
-gdImagePtr gdImageCreateFromJpegPtr (int size, void *data, int ignore_warning)
+gdImagePtr gdImageCreateFromJpegPtr (int size, void *data, int ignore_warning, int prescale)
 {
 	gdImagePtr im;
 	gdIOCtx *in = gdNewDynamicCtxEx(size, data, 0);
-	im = gdImageCreateFromJpegCtx(in, ignore_warning);
+	im = gdImageCreateFromJpegCtx(in, ignore_warning, prescale);
 	in->gd_free(in);
 
 	return im;
@@ -289,7 +289,7 @@
  * Create a gd-format image from the JPEG-format INFILE.  Returns the
  * image, or NULL upon error.
  */
-gdImagePtr gdImageCreateFromJpegCtx (gdIOCtx * infile, int ignore_warning)
+gdImagePtr gdImageCreateFromJpegCtx (gdIOCtx * infile, int ignore_warning, int prescale)
 {
 	struct jpeg_decompress_struct cinfo;
 	struct jpeg_error_mgr jerr;
@@ -327,6 +327,8 @@
 
 	cinfo.err->error_exit = fatal_jpeg_error;
 
+	cinfo.scale_denom = prescale;
+
 	jpeg_create_decompress (&cinfo);
 
 	jpeg_gdIOCtx_src (&cinfo, infile);
Index: ext/gd/libgd/gd.h
===================================================================
--- ext/gd/libgd/gd.h	(revision 289063)
+++ ext/gd/libgd/gd.h	(working copy)
@@ -247,8 +247,8 @@
 gdImagePtr gdImageCreateFromPngCtx(gdIOCtxPtr in);
 gdImagePtr gdImageCreateFromWBMP(FILE *inFile);
 gdImagePtr gdImageCreateFromWBMPCtx(gdIOCtx *infile);
-gdImagePtr gdImageCreateFromJpeg(FILE *infile, int ignore_warning);
-gdImagePtr gdImageCreateFromJpegCtx(gdIOCtx *infile, int ignore_warning);
+gdImagePtr gdImageCreateFromJpeg(FILE *infile, int ignore_warning, int prescale);
+gdImagePtr gdImageCreateFromJpegCtx(gdIOCtx *infile, int ignore_warning, int prescale);
 
 int gdJpegGetVersionInt();
 const char * gdPngGetVersionString();
Index: ext/gd/gd.c
===================================================================
--- ext/gd/gd.c	(revision 289063)
+++ ext/gd/gd.c	(working copy)
@@ -342,6 +342,7 @@
 #ifdef HAVE_GD_JPG
 ZEND_BEGIN_ARG_INFO(arginfo_imagecreatefromjpeg, 0)
 	ZEND_ARG_INFO(0, filename)
+	ZEND_ARG_INFO(0, prescale)
 ZEND_END_ARG_INFO()
 #endif
 
@@ -2407,7 +2408,10 @@
 	FILE * fp = NULL;
 #ifdef HAVE_GD_JPG
 	long ignore_warning;
+#if HAVE_GD_BUNDLED
+	long prescale = 0;
 #endif
+#endif
 	if (image_type == PHP_GDIMG_TYPE_GD2PART) {
 		if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "sllll", &file, &file_len, &srcx, &srcy, &width, &height) == FAILURE) {
 			return;
@@ -2416,6 +2420,12 @@
 			php_error_docref(NULL TSRMLS_CC, E_WARNING, "Zero width or height not allowed");
 			RETURN_FALSE;
 		}
+#if defined(HAVE_GD_JPG) && defined(HAVE_GD_BUNDLED)
+	} else if (image_type == PHP_GDIMG_TYPE_JPG) {
+		if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "s|l", &file, &file_len, &prescale) == FAILURE) {
+			return;
+		}
+#endif
 	} else {
 		if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "s", &file, &file_len) == FAILURE) {
 			return;
@@ -2460,6 +2470,10 @@
 
 		if (image_type == PHP_GDIMG_TYPE_GD2PART) {
 			im = (*ioctx_func_p)(io_ctx, srcx, srcy, width, height);
+#if defined(HAVE_GD_JPG) && defined(HAVE_GD_BUNDLED)
+		} else if (image_type == PHP_GDIMG_TYPE_JPG) {
+			im = (*ioctx_func_p)(io_ctx, prescale);
+#endif
 		} else {
 			im = (*ioctx_func_p)(io_ctx);
 		}
@@ -2493,7 +2507,7 @@
 			case PHP_GDIMG_TYPE_JPG:
 				ignore_warning = INI_INT("gd.jpeg_ignore_warning");
 #ifdef HAVE_GD_BUNDLED
-				im = gdImageCreateFromJpeg(fp, ignore_warning);
+				im = gdImageCreateFromJpeg(fp, ignore_warning, prescale);
 #else
 				im = gdImageCreateFromJpeg(fp);
 #endif
@@ -2533,7 +2547,7 @@
 #endif /* HAVE_GD_GIF_READ */
 
 #ifdef HAVE_GD_JPG
-/* {{{ proto resource imagecreatefromjpeg(string filename)
+/* {{{ proto resource imagecreatefromjpeg(string filename[, int prescale])
    Create a new image from JPEG file or URL */
 PHP_FUNCTION(imagecreatefromjpeg)
 {

Thanks for the patch. However you seemed to have missed my answer where I said that I have a patch already.

Sorry, my last comment has been submitted too quickly.

I won't use this patch as it breaks API and ABI backward compatibility. It is also better to provide more user friendly functions to get a resized image without worrying about which scale can actually be used.

The patch is also missing the tests if libjpeg7 is actually used. I won't support patched jpeg6 as there are dozen versions out there.

oh, i missed your earlier post that you do have a patch. and your points on jpeg 6 is very true. i was just working on the test script. i guess, i will get back to my work :-)

In my opinion, it doesn't make sense to implement this only for
PHP's bundled libgd, but rather it should be implemented in libgd,
generally. Therefore I've submitted a respective feature
request[1], and suspend this ticket.

[1] <https://github.com/libgd/libgd/issues/368>