If you need to embed directly into the report rather than print all the images at the end of the report, you first have to add an image element. If you are using secured attachments, you will need to set choose "Dynamic image" in the "Select Image from" option list, create an onRender script for the image that makes a REST API GET request to the appropriate object structure (one with the table you're interested in and also has DOCLINKS as a child table) to pull in the base64 data of the image, and set this.data to that base64 data within the onRender script, which BIRT will then render as an image that is viewable in all formats (HTML, PDF, etc.) I figure out the call in Postman first to make sure it brings back an image.
If you are not using secured attachments you should be able to pull the image source URL from the DOCINFO table into a Data Set element, choose "URI" from the "Select Image from" menu, and set that to the field from the data set that holds the URL.