Without making a huge drawn out post on it.
Yes, you should have the author's permission.
You can put in a text link to any picture you want us to view as well.
There are many online picture storage services that allow hotlinking of any photos they have. Flickr is an obvious one. You are pretty much free to use anything on that site and hotlink it direct in a post as long as you have the correct attribution on it. Flickr has pictures that won't allow that per the artist/photographer so you don't have to hunt for it, the below instructions will simply not be available to you.
To use Flickr pics:
Find the Pic
Click on "Share This"
Drop down choose "Get HTML/BBcode"
Copy and paste that into the post (you may have to go to "Source" view to do it, top toolbar far left)
You are golden that way, post as many Flickr images as you want. Feel free to share other sites that we can safely use content with!
Otherwise upload your own artwork or stuff you are allowed to post to your heart's content. I am not going to spend a lot of time worrying about it, I got code to write.
Bottom Line: I love pics, but I hate lawsuits. We are a non-profit site and I don't think any court of law will fine us for a few pictures but if I get the request I will immediately take down any photos without talking to the author of the sub. Any request to remove uploaded graphics will be honored.