/System/Library/SyncServices/BTConduit.bundle/Contents/Resources
Once you locate your current icon's tiff file, copy the name of it to the tiff file you want to replace it with. Then just replace the original with the one you just renamed.
Notes:
- The files you will be changing in the bundle are TIFFs. So in Photoshop (or your image editor of choice), you need to re-save your icon file as a TIFF as well.
- Some icon files may resist opening/importing into Photoshop but copying or screen capturing can get around this with acceptable results. The end file will be displayed very small in the iSync interface.
- If you are making a custom file, just copy one that's already there and use it as a size template.
- If you want a transparent image, save your file as such. Otherwise you'll get a colored square behind your icon.

