To resize the images using Photoshop, open the image and from the Image menu, select Image size...
The image size dialog box will open:
If your image is a landscape format, then set the width to 1400 pixels - as long as Constrain Proportions is ticked, the height will automatically be set.
If the image is a portrait format, then set the height to 1050 pixels; this time the width will automatically be set.
With the image resized, save it as a JPEG image with a quality of around 9.
If you don't have Photoshop, or want to resize a number of images simultaniously, then the easiest way is to use a Windows Powertoy. The Powertoys are "add-ons" for Windows which were developed after Windows XP was released.
There are a range of toys, some of the very useful for photographers (for example, there is a raw image viewer for browsing raw images from digital cameras); to see them all click here. However, for the image resizer click here.
When you click on the resizer link, you should be prompted with a message like this:
Click Run and the installation should proceed.
With the resizer installed, you can right click an image in Explorer and there will be a new menu option; Resize Pictures.
If you select an image for resizing and select the Resize Pictures option, then you'll get a dialog box like this:
You'll notice that the size we want (1400 x 1050) isn't available, to get that option click the Advanced >> button, the box changes to look like this:
I've selected Custom as the size and set the screen size to be 1400 by 1050. I've also ticked the Resize the original pictures option.