I saw the following in a question page on Stack Overflow:
Which has compression artifact as seen when zooming:
Could this be changed? It doesn't look too good.
closed as too localized by Aaron Bertrand, yoozer8, Hugo Dozois, tombull89, ChrisF May 20 '13 at 21:02
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That's an advertisement served to users with less than 200 reputation in questions tagged as windows-8.
The original image used is indeed .jpg: http://cdn.atdmt.com/ds/NMMRTSMGUWDS/130405_winstore/winstore_ph3_games_728x90_static.jpg
This image is not under Stack Exchange responsibility, they just sold advertisement space to other company (Microsoft in this case) and that other company is responsible for the image and the ad contents.
This can't be changed by us, but you can "downvote" the ad so it won't show again: