Due to some ninja technical differences between wiki and regular content, you cannot upload an image using drag-and-drop or other native solution on the wiki page. Instead you need to create a wikified attachment. You can use the wiki attachment button in the editor toolbar, or alternatively the in the Wiki's public navigation "Upload New File". Once you save it, you can reference the attachment with the wiki's FILE BB-Code.
We are working to support non-wikified attachments in a future version. It's not a simple drop-in integration.
I'm assuming the widget permissions are set up and the widget is assigned to the area, since you already have other widgets working.
Some widgets take context from the wiki page in order to show. If the context doesn't reach the threshold needed for a certain widget, it won't be generated. This varies per widget. For the similar content widget, it takes the title of the wiki page, and generates keywords from that only. It searches the forum for those keywords.
Table of Contents is automatically generated when using VaultWiki's H BB-Code. If you are using the XenForo tags H3, H4, etc, then you need XenForo 2.3 in order for those tags to work with the wiki TOC. If you have fewer than 4 headings on the page, you can force it with:
You can manually place a TOC with:
[TOC][/TOC]