A nice solution is to make a set of backups (e.g. prefixed backup-), rotate the original files producing a set of new files (prefixed e.g. rotated-), giving you a set of
- img-1.png
- backup-img-1.png
- rotated-img-1.png
- img-2.png
- ...and so on
The mv/cp tools [bash globbing] can only add prefixes, it's messy to take them away (it'd use parameter expansion, ewww...)
The rename tool allows you to use s/before/after/ substitution syntax (from the sed tool) to swap that safeguard prefix and overwrite the original files, so your overall process for a given set of pictures img-{1..n}.png would be:
for imgf in img-*.png; do mv "$imgf" "backup-$imgf"; done
for imgf in backup-img-*.png; do convert "$imgf" -rotate 90 "rotated-$imgf"; done
Note:
- you could use cprather thanmv, but then there's 2 copies of the original lying around (downside is you get concatenation of prefixes, "rotated-backup-...")
- rotation is clockwise (-rotate 270gets you 90° anti-clockwise)
- to track progress, add echo "Rotating ${imgf#backup-} ... ";afterconvertcalls (beforedone)
- for a more compact form (e.g. a set of numbered files), use some parameter expansion like - echo "$(echo ${imgf#backup-img-} | cut -d\. -f 1)...";instead
 - ( You can't remove prefix and suffix in the same bash param expansion hence use - cut)
 
Then after verification you've not messed up, delete the pictures by moving them back to the original
rename 's/^rotated-//;' rotated-*
rm backup-img-*