Workaround: Blogger Mobile Thumbnail

When creating a blog post in Blogger that contains images, Blogger will automatically select one of the images (usually the first) to use as a thumbnail image for your post when viewing over the mobile interface.  Blogger does not give an option to override this section.

The workaround that I use is placing the desired image at the top of the post and editing the HTML of the image to hide the desired image (style="display:none;").

Note: This doesn't work until after the blog post has been published.  Blogger automatically removes the hidden image at that time.

I found the instructions in this comment by Deepak Kamat on Stack Exchange.