We don't have this capability, but it's funny you should ask because we happen to be working on it. For now, I suggest you modify the attributes of your image files in your favorite image editing software -- outside of EnviroInsite. I happen to use GIMP, which you can get for absolutely nothing from www.gimp.org. I'm sure it doesn't have all of the bells and whistles of high-end image software, but it seems to be just enough for my basic needs.
I have a work-around while I work with the graphics library vendor. Draw a rectangle that covers the image. Set the rectangle's background to Solid and its background color to white. Now set the transparency to 75% and click ok. You can move the transparency up or down to change the effect on the appearance of the image file sitting below it.
