If you photoshop it's a lot easier...
Open a new graphic and make the size as large or larger than you want the entire picture to be.
Fill it with whatever background color you want.
Open each jpg you've downloaded from the internet
Drag the jpg layer over to the new graphic file and adjust the location so it lines up.
Continue to do that with each jpg.
Once they are all lined up you can trim the file or crop it so there are no extra large edges.