Placing images over or to the sides of your Adsense ads is a great way to help CTR. Of course, you will have to mind the Adsense TOS, but I know that this is something Adsense account executives recommend (and even create) for their clients. So, here are the left boundary start points for a 4 ad, 728×90 banner:

Looks like there is about 4px of padding around all of the text in the 728×90 layout:

1) Starts at 4px
2) Starts at 186px
3) Starts at 368px
4) Starts at 549px