generally that means that you downloaded a thumbnail rather than the 100% image.
you need to zoom the image...look for a + sign and then click (usually double-clicking on image will open full size anyway.
simply opening the image (from Photobucket for example) will only give you about a 65% or so size.
can also get blurry images if you don't download full size and then try to make it bigger after the fact.
hope these tips help