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My guess is that the blank images are coming images associated to google but which were blank images. For example, someone had a single pixel image linked to with an img tag with alt text google. When Yioop downloads content from the web, it download up to some file size. I think for that crawl it was 20 or 50 kilobytes. If the image was bigger than that it might show up as partially grayed out. The thumb nails are then generated by scaling the result to some fixed size. The size of what yioop download can be controlled under Page Options. Detecting non-useful images would probably require additional programming.
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Edited: 2018-04-13)
My guess is that the blank images are coming images associated to google but which were blank images. For example, someone had a single pixel image linked to with an img tag with alt text google. When Yioop downloads content from the web, it download up to some file size. I think for that crawl it was 20 or 50 kilobytes. If the image was bigger than that it might show up as partially grayed out. The thumb nails are then generated by scaling the result to some fixed size. The size of what yioop download can be controlled under Page Options. Detecting non-useful images would probably require additional programming.