If you have a large number of files with random names ( lets say images from different sources with random names) and want to rename them all systematically for some reason.
On Microsoft windows there a simple trick to do this.
Select all the files.
Right click on any file and select rename ( or press F2 for rename)
Then give a name like img(1).jpg
And you are done. This will rename all the selected files as img(1).jpg, img(2).jpg, img(3).jpg .......
Please note that this will work for all kinds of files and extensions, not just images ( I took jpgs just for an example to illustrate)