Converting an image to a PDF file with ImageMagick can be as simple as:

convert in.png out.pdf

For single images -page letter can leave big margins, which may not be desirable.

convert image stack to PDF

This ImageMagick command puts one PNG image per page

convert -page letter -adjoin *.png joined.pdf

-page letter

does not upsize images, so tiny images will be tiny on the page, one image per page.