If you want to stop contents showing on landing page, enable the custom_index plugin and look at the index.php file of it. You can see how the different contents are called on the index page. Remove those contents which you dont need. Also you need to edit mod/custom_index/views/default/canvas/layouts/new_index.php

Upgrading : See http://docs.elgg.org/wiki/Upgrading_Elgg . You will loss all the modifications you have made in the elgg core. Thats why is is told to make your modifications as plugins.

seem not to have any luck there

when i add an image everything goes hair wire

I just want to replace the existing image, stop the blog posts from showing and show recent members

If it just happened after you activated a plugin, disable it and see if the problem still happens. If not then disable all non core plugins and activate them one at a time checking each time to see if problem returns. When the problem happens again that will be the offending plugin, look there for your problem.

Good Luck