Image placement on blog posts

Hi,

I choose the Nirvana theme because the example front page had the photos the full width of the column/ post, however I cannot see where to amend this, mine are only showing up small and in the centre with large gaps to the left and right, I tried amending the size under nirvana settings and this doesn’t change the size at all? Preferably I would like the posts to just show the featured image with the post title across it in white with no post excerpts and then have the option to click on that to read but I do not know if this theme does that and if it does how? It mentions having different themes for blogs etc but I cannot see anywhere where I can change this?

In addition if I add a header image it seems to centre itself so it goes under half the writing for the blog name and description but leaves a big white gap to the right, I cannot see anyway to just move this to the right so it fills the gap and doesn’t show under the header writing. I do not know css coding to work this out myself.

Thanks

Nicola