You have to have some sort of photo editing software. You load your pics into your program and then do the editing there. Save it to your hard drive and them upload them to fotki.
I use PowerPoint. Import the two photos, place them side by side and then save that slide as a jpeg file.
If you doing have PowerPoint or Photoshop, go to www.tucows.com and do a search to find photo editing tools. You probably will have to use the word image instead of photo in your search.