Okay, so here’s my deal with reposting fanart.
Source it.
Have this in your caption?

Cool! Power to ya for putting that in the caption.
Put a link to it instead, like this?

That’s cool too! As long as it leads to the actual original artwork, like so:

They’re perfectly equal, as far as I’m concerned. Hell, do both, if you wanna.
Know what’s not cool, though?
Reposting unsourced fanworks.
Imagine if you worked really hard to make something, such as a graphic, fanfic, or even just plain old fanart–and someone just posted it on their blog without crediting you.
You’d be mad, wouldn’t you? I make a lot of graphics for my other fandoms, and I’ve had to watermark my things for fear of people taking credit for something they didn’t do.
Sure, a lot of people who repost aren’t trying to take credit. But some people do try to.
Be a cool dude, and just credit things you use! It only takes a few seconds, and it makes you hella respectful.