Whenever a file is linked to with a size specification, e.g.
[[File:test.png|thumb|123px]], a new thumbnail is generated in that
particular size, and saved to the disk.
This is generally a good thing, because it minimises the amount of data
the clients need to download without losing quality at that display size.
However, this is also an avenue for denial of service - someone could
create many links to different images with non-standard sizes,
intentionally or unintentionally, and therefore overload computational
(temporarily) and storage resources on the server.
Therefore, I propose an option which would either limit the number of
stored/generated thumbnails or limit their sizes to a particular set
(e.g. powers of two) - however, this should not come at a loss of
functionality.
Whenever an image link requests a size which can't be generated, for
whatever reason, either the next-largest or the next-smallest image is
sent, with relevant CSS styles to resize it in the browser. The decision
between next-largest and next-smallest would be governed by a
user-preferences option which would default to 25% i.e. send the smaller
image if the larger image is at least 75% larger than the target size
(this should probably use the thumbnail area for comparisons rather than
the width, if it's not a major performance hit).
This proposal is especially important for public deployments with large
amounts of (especially non-technical) users and/or tight limits on disk
space.
