The slide show proceeds in name-sorted order, so you may want to force your desired order by prefacing your file names with numbers. A "slide" will be created for every group of files with identical base names. For example, this presentation had the following source files:
0010_hello.gif, 0010_hello.txt, 0020_continuing.gif, 0030_goodbye.gif
To create a slide, you MUST have a picture. However, you don't need a matching text file.