If you are rendering tifs with alpha channels, there should be no need to be messing around with brushes to blur or soften the edges. However, if you are rendering with Poser 4, you only have the option of turning antialiasing on or off. P4's antialiasing is far less than satisfactory, to say the least, so there are likely to be some jaggies. One option is to render at twice the required size, take it into PSP and resample it down to the size you need. In the layers palette, promote the background to a layer, load in your selection from the alpha channel and delete it. Then composite in your other layers or whatever you need to do. I think P5 has the option to manually set the amount of antialiasing? If so, crank it up more to smooth those edges. I create composite images all the time and have never needed to use brushes to get rid of jaggies.