I have been attempting to print 4" x 6" photos from my Fedora 15 install and the images (which I have edited with GIMP) have terrible pixelated edges near any fine detail where I have selected the surrounding region and filled with another colour.

I was printing to photo paper the XCF or PNG format files and they both were horrible.

So I thought I would fire up a Windows XP VM and print using the windows driver and suprisingly I got the same result.

However if I save the XCF or PNG file in JPEG format and then print on photo paper using Linux the quality is fine.

Seems like the HP driver on both Windows and Linux prefers JPEG format.

I have a suspicion this is because of layer information in XCF and PNG causing issues but I'm not an expert with image format and printing so that's just a hunch.

So in summary
If you are having trouble with image quality when printing to photo paper change the image format to JPEG it may help you.