You can include an image from a dashboard in a Conga Composer merge document by using an IMAGE: field.
IMAGE:fields are supported in Word and PPTX templates. (The instructions below are for Firefox -- it's a lot easier to accomplish this with that browser.)
Open the Dashboard. Right-click on the dashboard metric that you want to use. Choose ”Copy Image Location.”
Create a long text area field in Salesforce in which to store this value.
The value you copied in step 1 can now be pasted into this long text area field on a record.
Use an IMAGE: merge field to grab the value of the URL.