There are several ways you can create your thumbnails manually, for example you can just use any photo editing software out there and manually resize the resolution of each image. However, I prefer using Adobe Photoshop to produce a batch file that is like a mini program that has built-in instructions to rename image files within a designated folder accordingly to the settings that you have pre-set.
Try This Link:
http://www.creativemac.com/2002/04_apr/tutorials/psbatch.htm
If you wish for a simpler method, perhaps just googling the terms: "creating thumbnails" in google, and just download the variety of softwares that are free and allow you to make batches of thumbnails of your images.