I thought I would open up a thread dedicated to compiling tips for finding an image source, which hopefully someone more experienced with wiki edits will put into an article.

Anyway let's begin,

1) - Make sure that an image source isn't already on another related post (usually this will be by the same artist). If you do find one feel free to add it to the image you were just looking at though.

2) - It's always worth bumping an image no matter how old, with a source request comment, plus you could end up reviving an epic thread (Note -- DO NOT DO THIS MORE THAN ONCE PER IMAGE - You will be doing more harm than good by spamming 'sauce' 'sauce' 'sauce' again.

3) - Google is your friend, if you want to find the image source this is gong to be very, very useful, if your getting a diverse spread of results, you'll want to narrow down your results with words like furry, art, etc.
3.1) - Most useful thing to try is always the artist name.
3.2) - Failing that the name of the character.
3.3) - Lastly you can drag the image into the search bar and google will look for similiar ones.

Of course you'll want to take off safesearch in most cases, and you can always repeat the method with site specific searches like this site:[name of site] [what you want to search for] execpt of course without the square brackets.

4) The final method is to go to other boards/forums that permit furry content (Fchan tends to work best) and put in a request there under new threads. Just remember to always read the site rules on posting, and if you dropping by any of the 'chan' boards with a number in their name, please don't mention us by name.

Anyway, sorry if TL;DR and/or me sounding like I'm far more of an expert than on this than I actually am, and can someone please sticky this temporarily? Since this seems to be a pretty under-adressed topic.

There's also tineye and saucenao.

therabbidwanker said:
3.3) - Lastly you can drag the image into the search bar and google will look for similiar ones.

O.O
I had no idea that this is possible.

BranislavDJ said:
O.O
I had no idea that this is possible.

News to me, too.

BranislavDJ said:
O.O
I had no idea that this is possible.

Yep, it's pretty cool, although most of the time it just returns images that look similiar.

This thread should have happened much sooner.

People posting pony stuff seem to be especially lazy with sources, which can usually be found in a couple of minutes using ponibooru or bronibooru.
For things looking like the come from pixiv, danbooru seem to work reasonably well.
And of course searching Pixiv or deviantArt itself
That pretty much cover everything for me, if tineye and google image search fail.

Also, link to artist's main page instead of the image, especially for FA and Pixiv.

When I'm looking for a source...

- First thing I do is search for the image with TinEye:
http://www.tineye.com/

There are TE plugins for various browsers.

- If that doesn't work, I use Google's reverse image search:
http://images.google.com/

There's a camera icon on the right side of the search bar. Clicking on it opens Google's "search by image" feature.

- If I suspect that the image is from Pixiv, I use SauceNAO:
http://saucenao.com/

- If the image is likely to be found on one of the many anime image boards, I go to:
http://iqdb.org/

- Non-pornographic, western-style art can often be found on deviantART:
http://www.deviantart.com/

You can search DA by image dimensions. "222x333" (w/o quotes) searches for all images with those dimensions. If the list is too long, you can add one or two keywords, like "222x333 furry".

- Google's advanced image search:
http://www.google.com/advanced_image_search?hl=en

Set "image size" to "exactly" and enter the image's dimensions.
You must enter at least on keyword, or the search won't work.
Keywords like "furry", "anthro", "yiff" and "hentai" have been known to work.

- The Wayback Machine:
http://www.archive.org/

edit:
It's too bad that FA doesn't let you search by image dimensions, or at least by aspect ratio.
It's also unlikely that they'll ever add this feature, because it'd be useful.

edit2:
Don't underestimate the power of searching by an image's dimensions.
Width and height are almost as good as a unique signature, because many artists use weird dimensions.

Another tip.
Find an image's MD5 hash via tool(like implbits's Hashtab) or URL(Most imageboards save image's hash in filename) and search other imageboards for it, like gelbooru, rule34.paheal, etc.
The syntax is md5:%hash% or hash=%hash%.
There is a chance the image might already be on another imageboard and possibly have artist/source tagged.