How to easily insert frames at the end of a video?
A quick Google search for the terms add photo with virtual dub shows this result. Quoting:


  
  save the titlescreen in bitmap (same size of video, obviously)
  import image in VirtualDub (with Open command)
  select the single frame you get after opening and copy it
  open your video
  paste the frame you copied in the desired position (every ctrl-v is a single frame, if your movie is 25fps, just paste 25 times to create
  1-second title sequence)
  


For your case you don't have a titlescreen but just a regular image.

You can select frames in VirtualDub as following:


Click on the timeline on the location where your frame that you want to select. In the image below I clicked on frame 30.
Click on the button where the arrow points to. This sets your beginning of the selection.





If you want to select one frame, click on the 31st frame (as in the image below) and click the button that is indicated by the arrow. This sets your endpoint of the selection.


 

You can also select a series of frames this way.