Yes, once your picture has been chromakeyed, you can drag it around using your mouse. It is also possible to edit the used backdrop name to make the picture appear at a given default position on the backdrop. To do so, you edit the used backdrop file filename. Goto Preferences and click on the view BackDrop folder to open the backdrop folder, select the backdrop you would like to edit, and change the name to set the position. This is done by using _ (underscore) to separate parameters;
For example, if you want a picture to appear at 70% of the width of the backdrop, and 30% from the top, with the face taking up 10% of the height of the backdrop, change the name to this;
So separate the name and the horizontal position by 2 underscores, and the height and size by a single underscore.