Workflow 1 - It is not compulsory to include the image in the master page. Just create a new layer and add background images to it on the page itself and keep images layer below the text box of page number.
workflow 2 -
"does that mean, I should include two layers in the master page - the bottom layer being the page number and the top layer being a frame for an image."
Yes you should include the master image in different layer and text page number on different layer. But remember image layer need to be below the text box of page number.
Both the workflow are fine but if images are of same size and need on each page than its better to include the image placeholder inside the master page itself.