yeah u can Joe, i just tried. If there's a layer of an image you want to control the transparency with a null, open that image's layer to reveal it's Opacity, and ALT click on the Opacities Stop watch...then pickwhip to the Null's Opacity and you will be able to control the image's transparency with the Null.
You can pick whip the opacity attribute of the layer you want to control to the opacity attr of the null.
1) select the null layer and the layer you want it to control and hit T to expose the opacity attributes.
2) alt+click the opacity stopwatch of the the layer you want to be the slave (The opacity value should turn red & the expression controls will be revealed)
3) click+drag from the spiral to the opacity attribute of the null. (in the slave layers expression field you should see something like "thisComp.layer("Null 1").transform.opacity")
4) click off of the layer to exit the expression field.
5) if you want to disconnect it alt click again on the slave layers opacity stopwatch.
Thanks a million, it worked, i've always stayed away from Expressions but with your explanation i would be using these more. again thanks
happy to help