The following two images went through the same scaling code in zoom. The first image uses a 'mitchell' filter which trades ringing for smoothing to try and get a generic catch all filter. The second image uses the filter 'point' which just point samples the source image no filtering at all. Making It Fit, Pt. Here's another example.
Chances are that aliasing was introduced when the image was rescaled.
I like to browse lots of space sites like NASA's image archives and find big astronomy images for my PC's desktop. Zoom is great for resampling those images to fit nicely on your desktop. I found this HUGE NASA radar image of Salt Lake City , at 1494 by 2500 pixels. Now normally JPEG would be horrible on this kind of image because it would ditch all the high frequencies and introduce artifacts. Because we are downsampling, the filter will smooth out the high frequency components that will give JPEG trouble, so when we convert to JPEG, the fine detail of the streets and causeways in the Great Salt Lake are preserved as best they can.