SEO and "Post Only" websites

I’m going to be building an evergreen website with this theme: http://www.elegantthemes.com/gallery/origin/.

However the support, unhelpfully, won’t assist with telling me where in the code I can change the home page thumbnail images from posts to pages. This means that i’ll probably have to leave “as is”.

If I build a website entirely using posts, will it affect seo dramatically?

I know – strictly – that pages are the most appropriate format to use for evergreen websites, but this may not be possible.

Thank you!