A while ago, their ad network started to serve .apk to Android users (see https://pay.reddit.com/r/Android/comments/1qwb2i/weird_apk_t...). This is way worse than "just a fullscreen ads", since a malware on Android can easily steal your contacts and make you pay a fortune using premium numbers.
Their ads provider are probably doing this without the consent from imgur, but I doubt they'll change ads provider. After all, the one serving the more shady ads are the one paying more.
I somewhat doubt it's malware on my computer, because it only happens on imgur.