First, the size of the shape will affect the quality of the image, smaller is better of course. Are you sure you got the sizes set up the same in your V7 vs. V8 comparison?
As to a workaround, have you considered using BG Image map with a very large transparent mark on it to take your actions. Here's a step-by-step guide for using BG Images on a "map".
Thanks for the suggestion - I had totally focused on shapes and forgot that you could use images as backgrounds to tables (and then trick it not to show the table). It should work just fine.
I created each workbook from scratch and I used the same JPG for each workbook. The size in the v8 workbook should be very close (if not spot on) to the size in the v7 workbook.