There is not an easy way in ArcGIS 10 scripting to set the symbols of a DRG image. The best way to make the background symbol of a large number of DRG images transparent is to manually make the background of one image transparent and then run a short series of Arcpy commands to copy the symbology to the other images:

1. Make the background of one image transparent:
   1. Double click the image in the table of contents. This should open the layer properties window.
   2. Double click the white symbol in the second row on the Symbology tab. This should open the color picker.
   3. Click on No Color and then click OK in the layer properties window. You won't notice any difference, because No Color will look white.
2. Click on Geoprocessing in the top menu and select Python. This will open the Python window.
3. Copy and paste the following code into the Python window. The code assumes that the image you fixed by hand is the first item in the table of contents, whose index number is 0. If it is not the first item, change the 0 in the brackets in the "fixed" line to one less than the number you have to count to get down to the item you fixed.
4. You have to copy and paste the lines one at a time. The Python window does not work right with multiple lines copied from a web page. Press enter twice after pasting the last line and the white color of all the tif images in the data frame should become transparent.