What many other sites as huge as yours (and that I also do) is run a CRON task on the server every night that looks at all the files in the attachment directory that are less than 24hrs old and runs them through a resizer to;
1. Reduce to 1024px MAX width/height. Skip smaller images.
2. JPG/JPEG compression quality to 90%
3. Remove all EXIF metadata that is not related to time/date/camera specs.
4. Keep file creation date.
5. Overwrite original.
this is what i do as well and since the site is so busy, i run the cron every 4hrs ( choose your poison ). i additionally, make a thumb-whateverimagename.png for each one, so a [thumb] UBB code can display that in places where a thumbnail might be preferred.
the main problem i ran into was that all files were dumped into 1 directory, so i modified the attachments dir to have sub-directories named starting by year (yyyy), then sub-dirs under each for month ( 01, 02.... 12 ).
that makes it easier to not have one directoy with a HUGE number of files in it.
