We have talked a lot on meta about copyright in answers, but so far it has seemed to only cover copyright of words and text, not of images.
How do we make sure we are citing things appropriately? Do we even have to worry about images on this site?
Okay, so first of all, we most definitely need to make sure we are aware of how copyright pertains to images we use on our site (and the internet as a whole, but that is a whole other kettle of fish).
The first thing we need to note is that every user who makes an account here is bound to the same Terms of Service. One of the most interesting parts of the TOS as it relates to this question, and others relating to copyright, is the part that states:
"Subscriber represents, warrants and agrees that it will not contribute any Subscriber Content that (a) infringes, violates or otherwise interferes with any copyright or trademark of another party...(c) infringes any intellectual property right of another or the privacy or publicity rights of another"
Which in essence means that each user is responsible for policing their own content and making sure that they attribute it appropriately, and it is not SE's job to make sure that it is being done.
While that is all well and good, and makes perfect sense for an organization with this many users, how can we as users make sure that we are using images appropriately?
First, the easiest way - is it copyrighted in some way? Is the site you are grabbing it from copyrighted? In that case, you likely should avoid using the image, as you are in all likelihood breaking copyright (although there are many exceptions to this rule), and that is something we definitely don't want to encourage.
One of the best ways to find safe images to use is to use sites where their image repositories are listed as being free from copyright (or listed as copyleft). You can google search for these sorts of places, and while they might not have the exact image you need, they are safe for use.
Another thing you might run into are images released for use under a particular Creative Commons license. There are a number of these licenses, and they all have different requirements on how the work can be used, and how it needs to be attributed. I am not going to go through every licence they have, as I feel that the CC site does a great job of explaining how they work and what you need to do, but I will include one example.
The easiest one to attribute is the CC-BY licence. Under "Attribution", it states that
"You must attribute the work in the manner specified by the author or licensor (but not in any way that suggests that they endorse you or your use of the work)."
This generally means that you need to follow any specific way that the author has listed that they want to be attributed, and if nothing is listed, it is generally assumed that you will list the full name of the creator, and a link to the original host of the image. (Sometimes, finding the original source can be tricky, but it is a necessary thing. The "search by image" function on google can often be helpful here.)
In general, we as a site want to try to avoid breaking copyright, as it is unfair to the original creators of the images that we use them without them actually allowing us to do so. Does this mean we can never use images? I don't think so - I think there are definitely ways we can do this (uploading our own images is one way, as then they just become part of SE under their Creative Commons licence, and then we don't have to worry about accidentally causing problems, as we have given the site the right to have that image posted).
As with anything, we need to make sure we are using the information appropriately. :)
Ashley's answer covers this in fantastic detail, but to just add my 2 cents via TL;DR;
By the TOS, anything you post is automatically licensed via CC-Wiki. So if you do not have the rights to license a particular image under this license then you should refrain from posting it, and opt for another image that is in the public domain or that you have license too.