Using Pages '09 v4.3, you can combine two images, select both, group them, copy to clipboard, and select New from Clipboard in Preview. They are now in PDF format in Preview, where you can save, or export to other formats, or copy/paste back into Pages as a PDF.
I reference the older Pages, because in Pages v5.6.2, your trip from Pages through the clipboard to Preview produces a 72 dpi PNG graphics file — not PDF, and not the JPG of the two combined images.
If you don't have Pages '09 v4.3, you should combine your images in a dedicated graphics program (e.g. GIMP (free), Photoshop, Affinity Designer, etc.).
You cannot combine images in Preview, as you can PDF. The image combination appears to work until you save and review that content, to find that it didn't work afterall.