In Encore, you have the option to Build to Disc, Image, or Folder. Like you, I use Image since that gives me a file on the hard drive from which to burn more copies later, good workflow. One can choose from many FREE disc burning utilities for this task, or you can do it from within the Encore Build window.
Just tell Encore to "Build to Disc", but then for the Source, which by default is "Project", change that in the drop down to "Image" and then specify the file path for the .iso file you'd made earlier.
I don't have Encore to look at right this moment to be more specific, but it is pretty simple really when looking at the Build window options.