When adding multiple files to an animation, rather than listing every single filename, you can use masks. For example, *.png or walk*.bmp.
Masks cannot be used for the /O or /S switches, or the implicit versions of these.
Gif Animator uses natural sorting when processing the results of a file mask to ensure the files are processed in the order you expect. Therefore, for example if you have the following files in a folder:
file1.png
file2.png
file10.png
These will be processed in the order you would expect (file1.png, file2.png, file10.png), not the way your OS sees them (file1.png, file10.png, file2.png).
Note that this ordering only applies when using masks, otherwise ordering remains as specified on the command line.