If you are using photoshop go to Image - Image Size and enter the size you want.
If the file is too big in size go to Image - Save for web.
Up comes a new screen.
Use the slider to reduce toe file size to what you need
If you image is called something like Beach.jpg or what ever is a good idea to save the thumbnail as something like beachth.jpg.....just putting th on the end reminds you its a thumbnail.