Edit: Disclaimer: I do not animate; any questions about animations and previews will be answered with a best guess

The last time I ran a projecteducate week, I tackled preview images.  So this is an up to date, better informed version of the old 2010 article.  Like the last one, I will cover when you need a preview and the dos and don'ts.  New to this issue will be myths surrounding preview images and why preview images are needed.

When are you required to use a preview image?
Here on dA, whenever you upload a .gif image larger than 150x150 pixels,
you will be prompted to add a preview image that is 150x150 pixels or
smaller.

Why do you need a preview image for .gif files?
"GIF is a complex format which may contain animations and does not automatically resize well. We require the previews so that you get the chance to best represent your artwork." - ZombieCoder  Basically, a .gif is a .gif, so even if it is not animated, it can be re-sized awkwardly, so preview images are required even with static .gifs.

My .gif isn't animated and I don't want to use a preview.  What can I do?
You can save static pixel images as .png which will keep your colour count intact as well as transparency.

Doing Preview Images Right
Take a good look at the image you are submitting.  Try to find the most  interesting section of your image, the section that you think is going  to draw attention to your work.  That is the section you want to use to represent your piece and make a 150x150 preview of.  I recommend using the full 150x150 because it will show more of your work, which makes it less likely to be overlooked.
:thumb202140306:

Some deviants who know that they will be submitting .gif images frequently have created their own personalized frames/templates for their preview images, which helps to make them look clean and recognizable.

Mature Content

Doing Preview Images Wrong
The biggest do not in my books is: do not use the same preview image for every submission.  Some of you might wonder why not to use the same preview image every time, and I can explain.  I used to do this, about 4 years ago.  I simply had a 150x150 black square that said "preview" on it in red letters.  I thought it was logical and time efficient.  I then started watching someone who did the same thing and I realized that I didn't like seeing it; I didn't want to click on something with no clue as to what I was clicking on.  I even sometimes thought that the person had submitted the same deviation twice!  Long story short, I saw how confusing it could be to my watchers and how it was affecting whether people chose to view my submissions or not.

Preview Image Myths
"Preview images are just a cheap trick to make people look at your work!"  - In this case, every thumbnail on dA is a cheap trick.  Previews are required for some file types, .gif included and optional for literature.

"I can use any image as a preview!" - Nope, only your own work FAQ #63: What sort of things can I use for Preview images?

"It's unfair that Pixel is singled out for needing previews!  It makes people less likely to look!" - Actually anything animated (flash submissions, traditonal and digital animations), .gif images and installable journal CSS code all need preview images.  And while a few people are deterred by preview images, it makes others more curious, so the amount of people that see a work is generally unaffected

And that's the sum up of preview images