Graham bemethavinci I presume you have a copy of the standard twig
The blog twig is at https://dl.dropboxusercontent.com/u/2162515/xtgem_blog.twig
you upload by url to _xtgem_templates and rename it xtblog.twig
Some of what you want to do is not simple the image is part of the {{ entry.content|raw }} so to get the layout you drew the image has to be extracted from the entry content
The basic layout part for the post list is like this (no changes to the actual twig code yet just to the embedded html for the list so it creates the post containers)
The actual twig for the example is shown in the second panel on my test page
http://baretest.xtgem.com/00single
next I will look into extracting the image from the description