How can I stitch a set of images together?
You can do it with Imagemagick from a terminal. If you have Homebrew, installing it is as easy as:

brew install imagemagick


Once you have it, open the terminal and go to the folder where you have the pictures, then do:

convert foo* +append result.jpg


And there you have it, your 100 pictures in a row. You can also make a column with them, instead of +append use -append.