I have a sequence of images (PNG in this case). They are part of a folder # 43 and they are named 43_01, 43_02 etc. Up to 43_99 everything was OK, but when I created the image 43_100 the program put it right after 43_10, then followed 43_101 etc, overall the the images were ordered like this:
43_09
43_10
43_100
43_101
43_11
etc
Hope this helps.
Genko Velev