Depends on what you use them for. Services like ImageShack and Photobucket keep your images around, while (last time I checked, which is admittedly about a year ago) Imgur's FAQ says something about deleting old images that haven't seen any traffic for a certain timespan (at least 3 months, IIRC).
Glancing at the source code http://mikescoding.com/imageshack/index.phps for 30 seconds, it seems the way this works is that the uploader IP address is retrieved from some XML file on the imageshack servers. It seems every image on imageshack has a corresponding metadata XML file stored at a secret location, but the algorithm to calculate this URL was exposed during the earlier pastebin leak?
The XML URL is calculated by taking the image's filename (minus the extension), calculating the MD5 hash of that + a static salt (which is visible in the source), then replacing the image extension with the first 10 chars of the hash + ".xml". Example [1]. What type of third party would they be giving this API to?
They 403'd this specific link it looks like. An image I uploaded about an hour ago seems to be working still. [1] is the data it returned for the 403'd image. Here's [2] some Ruby code if you want to try it out yourself
I could be wrong, but isn't the precedent that IPs aren't adequate evidence of a person's identity? I don't think that a dynamic IP address that was used 18 months ago is going to be of any use now.
Possibly by it being linked from a torrent description saying "Here is this movie that I pirated and am offering to you illegally for free - <username>" and then having that same username attached to the ImageShack screenshot that also provides your IP address?
Plus, if we're being pedantic, most pirated movies shared online are re-encodes of their original source material.
Given the sheer number of variables involved in the encoding process, the odds of independent encodes being absolutely identical are extremely low.
It is trivial to determine from a screenshot whether it is taken from the source or a re-encode (i.e. whether it is from a legitimate source or not, assuming the source is the only authorized copy), and if you have a copy of the particular re-encode, to verify the exact provenance.
Here's what their compete chart looks like, for what little it's worth (login required, so screenshot instead): http://cl.ly/image/3z1v152G3r1l