I use the plugin Thumbnails for Excerpts. To make it work for articles with videos rather than graphics I had to go outside the box a bit.
I take screenshots of the videos, make them 150px, put them at the bottom of each article, and sized them to 5px, so they look like a dot ending the article.
It has all worked well with the themes I have tried. But with Easel, which I have decided is one for me, when I do categories or search or archives it generates two thumbnails in each excerpt.
What it is doing is resizing the 5px back to 150px.
It does not do that on the index page.