Add element to widgetpage

You can use @kudratullah’s suggestion to hook into the widgets page. The downside is that this allows insertion of a piece of html only in one specific place. If that location is ok, use that solution.

Otherwise, there are two other possibilities.

The first is to use jquery. Examine the source to identify the html element where you want to add your image (let’s say #wrap). Then append the node like this:

$( "#wrap" ).append( "<img src="https://wordpress.stackexchange.com/questions/252709/...." />" );

The second is to use the css after pseudoclass like this:

#wrap:after {content:url('/path/to/mypic.jpg');}

Both style and script files need to be enqueued to the admin.