How to merge two JPEG files back into a single JPEG (Both sides of a receipt in single image)
Process followed

select both source JPEG files in finder
Open them together in preview app
Print from preview app to see the print option dialog box
Configure print layout to output 2 pages in single sheet
Select &quot;Open PDF in Preview&quot; from the PDF dropdown menu (a new preview document opens with both source images in a single page)
Save this untitled document as JPEG

I hope this helps. Feel free here to post any other way than this, that might help.

update: For more technically savvy users, using imagemagick :)
convert image1 image2 +append result