Unless the link is an article or a wiki page of some sort which explains the topic better (a.k.a. "Also See"), but only an image to illustrate/example an idea, rehosting it on imgur is an easy and appropriate solution.
Directing links to external sites may have unintended consenquences against people whose providers (for example) have blocked one or another manga site. Imgur is neutral grounds, we haven't received complaints about images from imgur not loading.
About referencing a chapter. I don't think we should provide links to external manga readers. Mainly because most of those are illegal hosts/scans. Reference the chapter/page without links, and trust the OP to find it himself in whatever way he sees fit.