I always had the auto-generated Identicon picture as my profile picture. Yesterday I edited some fields in my user profile and my profile picture suddenly changed to another Identicon.
I liked my old one. Is there a way to get it back?
Yes, change your email address to your old address.
The Gravatar auto-generated image is based on your email address, change that, and the image changes with it.
If you really, really liked it but want to use your new email address, you'll have to save the image and upload that image as a fixed profile picture instead. Change your email address to the old, right-click the generated Gravatar image to save it to your disk. Change your email address back and upload the saved image to your profile.