Dealing with Covers

HI:

When I convert an html manuscript to an epub or mobi, I select the cover art I want to use via the metadata window.

Can someone tell me if I need to save a separate epub for each distributor/portal I might want to submit the book to, in order to change out which cover is submitted? Apple, KOBO, Googlebooks etc. all have different size specs for the cover art. So, while the book itself remains the same, the cover art is different.

Thanks!