From a theory perspective alone, ignorant of Lemmy specifics: a database query can be made to list all cached images including a unique identifier for each image. Use this list to find each cached image.

Look at your cached image list and decide how you want to prune it. The most likely pattern for this system is FIFO, so prune the oldest cached data until you drop below the target disk usage.

In practice, you’ll likely use somebody else’s solution. Be sure to read the contents of their solution carefully to ensure it doesn’t move sensitive data to an externally accessible location or exfiltrate data directly.

The point of a digital signature is to announce that you made this document, as it exists at the time of writing. Once a change is made it should no longer identify as signed.

Most institutions don’t use this functionality, despite the usefulness of it. At present, I’d recommend using it for publicly distributed files to protect against bad actors publishing a document that pretends to be yours.

As for legally binding, ask a lawyer. Generally, things are legally binding if they’re signed by all parties. The specifics get funky, but a digital signature is a solid step for announcing that you did this thing at this datetime and a judge should recognize that if it comes down to it. Bonus points if all parties attach their digital signatures.