In wiki guides and explanations, try to use as many images as possible, but dont go crazy with them. Try to explain as many things as possible with one single image.
As an wiki admin, you can access the media upload by visiting the Special:Upload page.
To see list of all files that are already uploaded to wiki, visit this page Special:ListFiles
The list of already available files is particularly useful for those images that can be reused in different tutorials.
For example, you dont have to upload new file to show user how to access JomSocial configuration page as this file already exist.
Note - the image is resized for the purpose of this help. You wont resize images in your articles as you want users to clearly see what to do without clicking the image thumb.
You can also upload your reusable images, but try to reuse them as much as you can whenever is possible.
For every new article that will include some Back End image, use the images from Joomla 3. Make sure that name of your test site is JomSocial Docs as shown on the image above.
If you're about to post image regarding external source, please use alert like on this example
Using images in articles
To use the image in the article, you will need to know exact filename and the extension of file you want to use