http://social.msdn.microsoft.com/Forums/office/en-US/528fe1dc-1250-4bf3-a544-6afe656e55cc/excel-vba-for-mac-wont-assign-color-or-image-properties?forum=exceldev

"But Will It Work on a Mac?
A common problem that I hear about is Macintosh compatibility. Excel for Macintosh represents a
very small proportion of the total Excel market, and many developers choose simply to ignore it.
The good news is that the old Excel XLS file format is compatible across both platforms. The bad
news is that the features supported aren't identical, and VBA macro compatibility is far from perfect."

I agree. It's a bug so let Microsoft answer.
Bob