There's no universal solution to control cover display in this specific way. Reading systems have different methods to show the thumbnail for the cover image. RMSDK based devices/apps generally pick the first content for the thumbnail.
But you can do it in iBooks and Kobo apps by specifying different values for the covers in the metadata and in the guide section.
For the thumbnail, specify the metadata like this in the OPF:
Code:
<meta name="cover" content="cover-thumbnail.jpg" />
...and for the full size cover, add the following in the <guide> section of the OPF:
Code:
<guide>
<reference href="cover.xhtml" title="Cover" type="cover" />
</guide>
The above considers that you have the reference for the "big" cover image inside the "cover.xhtml" file.