The thumbnail was kept with very low quality (approximately 10-15, 99 close to lossless). I think the question is: "Why did this happen."
I can think of some reasons, but you will have to experiment. I assume that the images you submitted are real images (not converted copies, like PNG to JPG, I mean), and the command line is complete and describes the complete image workflow.
your ImageMagick setting is trying to maintain the estimated image quality. You don't set the quality explicitly (e.g. -quality 75 ), so the thumbnail gets the same quantizer setting as the original image. Suppose the source has a low quantizer, but you don’t see it because of the high-frequency component (the image is “noisy” due to scanning). With oversampling, the background loses its noise and becomes a smooth gradient that is absent in the source. And a smooth gradient is hell on low quantizers. Try to explicitly set the quality factor (40 - 99, 40 is better compressed, but chunkier, 99 is a very high-quality, but larger file).
There is some interference between the resampler and the Moiré template, which creates a scanner in the resulting image. This is less likely because I see a “wavelength” of about 8 pixels, which is not uncommon, and I don’t think that with so many images that you acquired, none of them had about the same size and proportion of this; which in this scenario should have caused the same behavior. You say that this did not happen, therefore, if this image is not uncommon for size, aspect ratio or source (for example, one of the few images scanned using a scanning scanner 600 in a batch), this scenario becomes rather unlikely. But if this is correct, then add Gaussian blur before resizing , and this should fix things: for example. -blur 2x2 .
there is bad juju in the file name, and for some reason it gets the ImageMagick shell to interpret the command "set the quantizer to its most complex value". REALLY unlikely (if the interpretation interprets part of the file name as an option, it should not interpret it as a file name, and the rest of the file name is no longer the true file name, resulting in a "File not found" error that we don’t observe. However less, if the original file name is something like "--progressive-swedish-music.jpg", try renaming it before showing the thumbnails .
In any case, I put my money on option number 1.
Another test you can try is to run the same command from ImageMagick (command line), not from PHP.