Alternative method: If you're using InDesign CS5.5 or 6.0, you can place the image and type on the first page of the publication. Group them together. Choose Object > Object Export Options. Check Custom Rasterization. Set the resolution to 300 ppi. Check Insert Page Break After the image.
When exporting the EPUB, choose Rasterize First Page for the Cover option.
Europe, Middle East and Africa