It's hard to say what's going on here. My first thought is that these are data-compression artifacts. Data compression is, very roughly, looking for redundancies in one's data and then encoding the presence of those redundancies. There are a variety of algorithms for data compression, both lossless and lossy. Lossless means that the original can be completely restored, lossy means that it can't. Lossy compression can easily outperform lossless compression, but it's at the expense of losing some of the original data.
Text files one always compresses losslessly, for obvious reasons. The well-known archiver "Zip" does lossless compression.
But media files, audio, image, and video files, one often compresses lossily. One tries to lose details that are difficult to perceive, but if too much is lost, one will see compression artifacts.
For image files, PNG is a lossless format. For real-world pictures, it does not perform very well, because the number of bits one needs varies as the logarithm of the amplitude of variation. Thus, even a low-noise picture won't be compressed by very much. But diagrams can often be compressed much more, because they may contain large expanses of single colors.
GIF is an earlier format. It is lossless, but it is limited to 256 pixel colors. Getting a full-color image into GIF form requires loss of color resolution. If one does so naively, it makes gradients look banded. The solution there is "dithering", alternating between colors to get appropriate average colors. But like PNG, GIF can do well with diagrams.
JPEG is lossy, and not surprisingly, it performs better than PNG. JPEG has an adjustable quality parameter, and making it high makes a high-quality image with a large file, and making it low makes a small file with a low-quality image. Low-quality JPEG looks blocky, something that gives away how its image-compression works.
Similar considerations apply for audio and video. Lossy usually does better than lossless, and the smaller the file the worse the quality.
