To do that create a Java Project with dependencies to modules:
<dependency>
<groupId>org.jbpm</groupId>
<artifactId>jbpm-form-modeler-api</artifactId>
</dependency>
<dependency>
<groupId>org.jbpm</groupId>
<artifactId>jbpm-document</artifactId>
</dependency>
On your Custom Field Type you should implement the getShowHTML and getInputHTML in order to return the HTML that has to be shown on screen here is were you have to generate your img TAG with the value that you receive as a parameter. You should cast the received value to org.jbpm.document.Document and when you got it casted you can get the download Link using the getLink() method or also the content (byte[]) using the getContent() method.
Once you've done it, put your module in the server classpath and restart.
After that you should edit / create your form, add the document field you have to use and edit it's properties, on the Field Type combo choose Custom Field, the properties form will change and a combo with all the available custom types will appear so you could select yours.
Hope it helps,
Pere