Should poster frame be rendering as the file icon?

I am self-taught in AE and must have missed something basic here.

I am simply trying to set which image (frame) will appear as the icon for a renedered comp.  I understand how to set the poster frame for a comp, and I get the desired result in the project window when the comp is selected. I "set poster time" while the timeline window is selected as well, but when my comp is renedered the icon is always the frame at 10 seconds on the timeline.

I've done quite a bit of research, but must be missing something obvious.

Thanks!