Added the case in which the sequence does not start from 0, but another file number. Also, added explanation how to call the function.
Added support for different types of images (extensions).
Merging James' comments
Merging James' comments, part2
Merge pull request #1 from grigorisg9gr/master

Add support for different types of images, case in which the sequence does not start from image '0', and raising exceptions in case of mistakes.