Full feed with partial post on main page. Any idea how to?

Any idea how to have full feed and partial post on main page?

Optional excerpt doesn’t work, must it be supported by theme? But anyway I want have each post on main page with one full picture and little decription with read more option
so I’ve tried <!–more–> tag, but it’s probably only for cutting feed [and I have checked full feed], but I don’t want cutted feed.

I was searching through forum, and I found only not resolved topics, documentation too, and on all those internets isn’t any helpful information.

btw: tototu.sk