tarek2
I think the problem is the name of the files. What C3 does to attempt a numerical sort is very simple, after you set the sort type to "numerical" it just tries to convert the names of the imported files, which are strings of characters, into numbers, using a built in Javascript method. Then when it has all the names converted to numbers, it does the sort.
For example it will convert the string "12" into the number 12. But if you feed it the string "Foo12" it just fails to do the conversion to a number, because it doesn't know what to do with the characters which are not numbers.
The easiest thing you can do is just name all the frames in your animation with numbers. Ej. 1.png, 2.png, 3.png, etc.