People often upload images from tumblr that are 500 pixels wide:
https://e621.net/post/index/1/width:500%20source:tumblr

It just so happens that preview images on tumblr have a fixed width of 500 pixels.
You can click on them and get a bigger version! (not always, but often)

Also, when you upload something from a direct image link, like this one:
http://24.media.tumblr.com/b5944a8e14c9f49f0f2a204e9eb55b73/tumblr_mresteKsFt1s69dj1o1_1280.png

Please change the source to the post link afterwards:
http://matospectoru.tumblr.com/post/58044601635/link-midna-in-color-d-kawaii-desu-xdd

Because there's no way someone else can take the direct image link, and reconstruct the post link from it.

Protip: Inkbunny

Images with a width of 920 pixels are often (but not always) preview images from Inkbunny:
https://e621.net/post/index/1/width:920%20source:inkbunny

Go to the source, upload the full version, profit!

Good stuff

Hopefully everyone sees this.

And I mean 'everyone'.