well, i thought i had resolved this, or at least found a workaround.
since wordpress popular posts was not showing my "first post image", i would delete all of the images and then upload the image i wanted as my thumbnail first, update the post, and then upload the other images. that worked once, but now i am still getting random thumbnails in my sidebar, rather than the first in the post or first uploaded image.
if you look at my most recent post, the second image is featured in my sidebar. i really wish it was the first image in the post/code, or the first image uploaded.