How handle different covers?

Hi:

Question: I have successfully created epub and mobi versions of my book. In doing so I have filled out the metadata and the only thing that differs is the cover.

When I start going to the different outlets with my epub or mobi, they all have different cover requirements. I have already gotten cover art in all the different sizes. For my mobi I used a cover in the size specified by Amazon.

For epubs, do I need to create a separate epub for each portal, the only difference being the cover I select for the metadata?

Thanks!:)