Is there a way to force Assets to redo transforms? Client has uploaded a number of severely badly lighted photographs for a 'staff' section of a site and these all have had a 'staffThumb' transform applied to it on the sites pages.

I corrected the lighting on the originals in Photoshop and re-uploaded those -- how do I make assets redo these transforms? Deleting the images in the _staffthumb folder doesn't do it. 

Any pointers?
I can't actually reproduce your problem but here's a couple of things to check and try:

Clear your browsers cache!
Did you re-upload through Assets with the same filename and click replace?
Purge all the caches via the Clear Caches button on the settings page
Update Asset Indexes on that same page
Failing those, manually delete the contents of the /craft/storage/runtime/assets and /craft/storage/runtime/cache folders
You can also try deleting the generated images manually in web/assets/..
If you're still weeping into your coffee try emptying the craft_assettransformindex table in the database

Of course, back everything up before you try all this...