It was absolutely a parasite. It would not have survived to grow to the size it did without Reddit. Parasites can offer a beneficial service to the host but cannot live without it.
It has since evolved into something possibly capable of existing on its own but only time will tell.
My guess is it lives for a few more years at best.
Incidentally, this is not what biologists consider parasitism. To a biologist, a parasitic relationship is one that benefits one while harming the other. Do you think that reddit was harmed by imgur? If imgur benefited but reddit was not harmed, their relationship was commensal. If they both benefited, their relationship was mutual.
To capture the idea that imgur was totally dependent on its relationship with reddit (but not vice versa), you would say that their mutualism was obligate for imgur but facultative for reddit.
Parasitism is when one party benefits while the other is harmed.
It's true that Imgur has to grow its product now to something bigger. I won't even give them many years if they don't do that for a product that simple and ephemeral. They have challenges here.
The parasite metaphor does not include the idea of being a "copy-cat"; parasites in the real world do not do the same things as their host. Indeed, that is the entire point of a parasite, in some sense; to take advantage of the host setting up various expensive systems to survive and just using them instead of developing their own.