We'll put in our few bits here:

We're no master artist, and can't really draw high-quality character pictures from scratch. Thus we tend to use others' creations. Sometimes we modify them to better fit our parameters.

When we do the former (use others' creations as is), we typically go on deviantArt simply because there are more reliable images there than most other places. When we do, we make sure to check a few things:
[list][*]is the image marked as "DO NOT USE"? If so, we skip it, no matter how good it'd fit our character idea.
[*]does the picture fit the parameters? (gender, age, race, species, birthmarks/scars/etc, clothes...)
[*]are there any logical fallacies? (pointy-heeled boots on warriors, armor that doesn't protect and/or restricts movement, abnormal physical features)
[*]do we like it?
[*]are there any comments on the image indicating that author doesn't want it used?
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If the image passes all those tests, we'll use it. But we'll also add a textual description to account for what the image doesn't or can't cover.

If the latter, we will do the following:
[list][*]Is the image marked as "DO NOT USE"? If so, nogo.
[*]Do we like it?
[*]What doesn't fit our concept on it?
[*]Can we tweak that on our own, or is it too much work?
[*]When we tweak such, we do our best to make sure that the final image is radically different from the original, usually through tweaking background and/or lighting. Often, we'll change the color on clothes.
[*]Finally, if we're satisfied with the result, we'll add a textual description.
[*]Afterwards, we'll try to contact the author and see if (s)he is ok with us uploading the image on a site like deviantArt.
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No matter which of the two paths we use, we always try to credit the author. Especially so if we're uploading an image we've modified to a site like DeviantArt. It might not look overmuch like the source image, but its still based on that person's hard work.

images that aren't tagged with something akin to "DO NOT USE", are generally considered to be public domain. But even so, due credit should be given. Oh, and when we use images here that are unmodified from the source image, we will link to the image on the source site, so that the author is possible to discern.