There is an ImageScan plug-in which can do this. It can also do Rotate, Contrast (don't know about opacity). It is somewhat questionable how well it is supported however.
Doubt this would ever be "built in" to FileMaker, but at DevCon 2008 I demonstrated a solution that manipulated images using the SmartPill plugin from http://www.scodigo.com
We demonstrated adding a drop shadow and rotating the image: both operations used php code called from the plugin to take the image apart, add layers, reassemble and then reimport into a container field. If you could find php code to change opacity / brightness I'm sure you'd be able to do the same things. Micah at Scodigo is also VERY helpful with these kinds of mods.
Hope that helps.