There is probably a proprietary program in the operating system of your computer that has the ability to manipulate the image. In Mac it is "Preview", Windows has a picture and fax viewer that has limited capability.
If you launch the picture from your desk top the computer will automatically open it with the necessary program. Once it does, look for the drop-down menus for "Tools" or "Options" along the top of the window or hover over any icons to see what they do. You should encounter an image edit feature or a re-sizing tool. Click on that and then change the pixels to 640x480. In the case of your photo (attached), it changed from 2.3MB to about 156KB.
Another option I use quite a bit is to attach them to an e-mail message. When you select the photos for attachment there will be an option to resize them for sending. In Windows OS, it's a check box, in Mac you can select small, medium, large, or actual. Go ahead and have them re-sized and attached then save them to your machine from the e-mail message instead of sending them. Drop and drag them to the desk top or just do a "Save As". I'll use this when I have multiple photos I want to re-size, it'll do 'em all at one time.