The easiest way is to open your PDF document in Preview, turn on View > Thumbnails (if it isn't on already) and drag and drop your image document into the thumbnail pane.
You can combine entire PDF documents this way, either dragging them from a Finder window or dragging the title bar icon from an open document.
You can also drag and drop thumbnails between open PDF documents in Preview, and you can create a new document from the clipboard if needed using File > New from Clipboard. (Note if you want to drag other pages into it you will need to save it as a PDF first.)
Not clear how to use preview to make a pdf with multiple images. I'm guessing that is something that is easy if you have to do it often? I'd probably reach for html or word. Probably word, first.