Screenshot on Nexus 1, Android 2.3.6, Android stock browser
Same basic issue as bug 32095 which covers MobileFrontend-specific UI elements.
Thumbnail images, math renderings, etc get used at resolutions matching CSS pixels, which traditionally matches physical screen pixels on desktop browsers and many older phones.
Many newer phones have higher-resolution displays: iPhone 4's 320dpi Retina display; 240dpi "hdpi" on many Android phones including the Droid, Nexus 1 and Nexus S, and the upcoming Nexus Galaxy will also have a 320dpi-ish display.
This causes the images to be upsampled, showing either blurry or blocky, and is particularly noticeable for diagrams with clean lines and math equations which are meant to look as good as surrounding text.
CSS media queries can be used to get the display resolution -- you WILL NOT be able to reliably guess that on the server side from User-Agent data --