This is my biggest frustration with Evernote.
I often want to have a thumbnail act as a reminder of the content of the Note to trigger my recognition of the Note while in the application AND to help me remember mentally. In the Note I might want to have other images - screen clips of graphs etc... So Evernote then uses that as the thumbnail and defeats me. Personal Brain is a nice example of a tool that allows you to pick your own thumbnail while attaching documents and images with arbitrary detail.
By the way, I also don't like that Evernote Android shows an iconised loudspeaker if the Note contains an audio clip rather than a thumbnail of any image that is embedded. Then you have a ton of indistinguishable snippets. Again, I'd rather have control over the icon.