How Do You View Several PNG Images In Preview With Continuous Scroll?
Ok, here's what you need to do:  


Open up the first PNG image in Preview. Make sure the side bar is showing.


Open the second image in Preview, and from the sidebar, drag the thumbnail image from the second window to the first window's sidebar.


Now, you'll notice that the title bar says "2 documents, 2 total pages." Continue dragging images into the first image until all the images are combined into one PNG. Make sure you save it. :-)