2) In Plex, you will want to have an Edit field on your panel that you will use to display the File. This field will need to have a length long enough to display the file path of the file. 200 characters is usually enough, but if you need longer you can have the field be as long as you want. Please note that the size of the edit control in the Panel Editor will be the size it appears in WebClient. So, set the size accordingly.
3) In the Action Diagram code, set the value of the field to the file path of the file relative to the Web Server. For example, if you have a folder in your WebContent folder called "MyImages", and in that have an image file called "myImage.jpg", you would set the value of the field to "MyImages/myImage.jpg". Then, do a Put on the field. The Put is what will update the file being shown in the panel.
Note: If you want to display files that are outside of your WebProject, you can set up a Context in your server.xml to reference external folders. It would look something like this:
<Context docBase="C:\MyImages" path="mywebproject/MyImages"/>
This would allow you to reference the images by setting the field value to "MyImages/myImage.jpg".