If you are doing this frequently enough to warrant the expense of a PDF Editor¹, then you can open the original PDF invoice in that editor, and then simply drag and drop your jpeg image where you want it. You then click on the deposited jpeg, and using the grips that now appear on it, you can resize the jpeg image to meet your PDF fitment requirements before saving the PDF.
Using a free trial of Nitro's PDF Pro², I just dragged and dropped a tiff (but could be a jpeg) onto a blank PDF document and could resize that image as needed to fit the PDF page. Both Nitro PDF Pro and PDF Pro Essentials can be found in the Mac App Store (not trials) by searching for Nitro, or on the vendor's website linked previously.
¹ Apple's Preview is not a PDF Editor.
² Nitro acquired Smile Software PDFPen and PDFPen Pro products in 2021