They have weird artifacts and inconsistencies. This one is really blurry, and note the random pixels jutting out on the right site of the expanded user card (probably due to the text that was behind it originally):
This one is pretty good except for the giant captions, which are aliased quite strangely and noticeably less blurry than the above:
In this one the star and arrows are blurry, the numbers are not, the text is suddenly fixed-width/unaliased and the assymetrical arrow looks like it was hand-drawn MS Paint:
I understand the blurriness in the latter image due to the zoom, but everything else looks unprofessional and hacked together. Consistency is the most important thing, I think — the images should be saved with the same quality and whatever else needed to make the text look the same, and so on.
PNG is normally lossless, as well. The first image linked (the blurriest) is 26 KB. I recreated a similar image with no blurriness or loss (apart from the captions, left as-is) and it's only 15 KB, so no excuses! imgur blurs it, though — you may want to consider not using imgur for these, or seeing if it's possible to prevent it from adulterating the image.