[feature req] convert pictures, allow for options

[jficz]

As a user and/or admin I'd like to be able to automatically convert incoming pictures in various formats to a "normalized" format which I'd define.

Not a problem to have multiple flows for different input formats.

The output format should have at least these options:

• "envelope" or a maximum picture size (i.e. scale everything down to a specified maximum size, like 5MPx or 3200x3200 or similar, keep aspect ratio, do not upscale)
• convert to a specified format (AVIF, HEIFF, WEBP, JP2K, ...)
• set desired target quality

or set specific cmdline for imagick or whatever tool is going to be used

[cwilby]

I'm happy for somebody else to build/own this feature, but not ready to build it / own it myself yet.