The cart uses the asp:Image to display images on the product list and product details page and the asp:Image does not support flash. You can however display flash on the product details page. To add flash on the product details page go to the product setup page and in the produuct description rich text editor click the html button and add the following code. In the code below, make sure you replace the "swfFileName" with the name of your flash file you have uploaded to your server:
<object classid="clsid:D27CDB6E-AE6D-11cf-96B8-444553540000" codebase="http://download.macromedia.com/pub/shockwave/cabs/flash/swflash.cab#version=7,0,19,0"
width="200" height="100">
<param name="movie" value="<% =swfFileName%>" />
<param name="quality" value="high" />
<embed src="<path of file>" quality="high" pluginspage="http://www.macromedia.com/go/getflashplayer"
type="application/x-shockwave-flash" width="200" height="100"></embed>
</object>