In August 2010, Stack Exchange started providing user uploads to an Imgur Pro account:
Alan Schaaf, the man behind Imgur, generously provided us a network-wide "pro" account that keeps any images hosted through our websites around indefinitely.
At that time, uploads to free accounts would expire at some point. But I just learned that since February 9th, 2015 all accounts are "Pro":
We’re phasing out Pro subscriptions...and making the Pro features completely free for every Imgur user!
I am not 100% sure this also applies to anonymous uploads, but their Help now says:
How long do you keep the images?
Sarah Schaaf - February 16, 2015 22:23
Forever! An image is only removed if deletion is requested.
Though some explicitly want to be able to control their uploads, I guess chances are low that an uploader requests deletion. But Waffles commented in May 2011, when the
i.stack.imgur.com URLs were introduced:
we are in the process of migrating to new dedicated image servers which we will back up regularly. Once this is complete we will announce. [...] Our main goal here is to have local backups of images you folk upload and dedicated servers
When editing, should we still move images from
i.imgur.com to the Stack Exchange account? (By using the "provide a link from the web" option from the Ctrl+G image upload dialog.)