Building the sidebar content on each node
There probably is a better solution, but this is what I thought of.


Make two new content type of 'sidebar-block-text' and 'sidebar-block-image'. And in those content types add a node (or entity) reference field. (Make sure to use the same field, so create it once, and for the other content type choose it from existing fields!)
Build a view-block.


Let it display the whole rendered node.
Add a contextual filter of the current node id and link it to the reference field. 

Place the block in the template sidebar region.


All the administrator will have to do is to add a new piece of content (sidebar-block-text or -image) and reference it to the 'parent node'.