Suppose I had about 500 .JPG and .jpg files in my folder "c:\images\". How to ask
Mathematica to load all the .jpg and .JPG file names then make a case insensitive sorted list out of them, say {"butterfly.jpg", "Fish.jpg", "marilyn.jpg", "Rabbit.JPG"... }, and finally rename the files in the folder as:

fnameLst = FileNameTake/@
FileNames[{"*.jpg", "*.JPG", "*.jpeg", "*.JPEG"}, "c:\\images", Infinity]