Lets say you have all your animation frames for your sprites in separate PNG files, like mustang_00.png, mustang_01.png, mustang_02.png, ..., mustang_31.png

You could then tell the resource the "pattern" of your filenames:

<sprite name="mustang">
<image fileseq="mustang_.png" start_index="0" leading_zeroes="1" />
</sprite>

This will try to load all the available images in sequence. It stops loading when it can't find an image with the correct number.