When an Imgur image is loaded from /r/4chan, imgur loads a bunch of images from 4chan's content delivery network or 8chan (unclear at this point, might be both), which causes a DDoS to those sites.
"Take a look at the network tab in the developer console when opening up one of the links. Over 453 requests made. Doesn't happen for other non-4chan images. Something fishy indeed:http://puu.sh/kjzzU/c926757f68.png"' 8chan XSS/imgur breach' gives an explanation,
tl;dr This is not for DDoS. This is an XSS against 8 ch, being spread through some sort of imgur compromise, and is very similar to another XSS against 8 ch found and exploited by the same entity back in January.
The attacker exploits XSS on 8 ch via Flash and probably a domain misconfiguration/oversight on 8 ch (SWFs can be uploaded by users + static content can be accessed through the "media." subdomain as well as the root domain). XSS places a persistent beacon on all 8 ch pages to wait for further JS to run, as issued by a server, though the new JS is yet to be sent.
XSS is spreading to likely users of 8 ch by compromising imgur through unknown means. No DDoS, no attempt to exploit recent Flash CVEs (yet). Flash is only used for XSS purposes here.
Mitigation: Visit any 8 ch board and type 'localStorage.favorites' in dev console. If you see a string containing a bunch of '\u0055' type numbers, then you fell victim to the XSS. Simply type localStorage.favorites = "" and refresh the page, and you're safe, as long as you don't load the compromised Flash again.
In the meantime don't visit imgur in the near future, and install a Flash blocker like Flash control, or a more robust blocker like NoScript or uMatrix.