How to put a static image in a flow designer ?

The Rich text editor on the screen element currently does not support img src tags.
However, the flow run-time interprets any text as HTML. So, you could store the html markup in a custom setting or any database record, fetch it in a variable and use that in the screen.

You can display document image in flow Designer by creating text variable.

Create Variable of type text in flow Designer.

Data Type: text
Input/output type :  Output only
Default value:   <img src="https://Domain_url/servlet/servlet.FileDownload?file=Your_documentid&oid=Org_id" width="100px" Height="100px"/>

Note: Domain_url  >>>  set your domain url
           Your_documentid >> set your document id.
          Org_id >>> set your org id

You can get your Org id by Navigate to Administration Setup > Company Setup> Company Information.

you will find org id at Salesforce.com Organization ID field.

Now use this newly created variable in Screen.