What are you using to edit your photos?
If you are using Photoshop, use the command Image>Image Size>
That will open a dialog box where you can resize your image - enter the pixel size for one side (usually width for a landscape image) and leave the "Constrain Proportions" box checked - that will allow PS to keep your aspect ratio, automatically setting the size for the other side. For example, if you have a photo cropped landscape at 3:2, and enter 1024 in the width, PS will resize the height to 683. Coincidentally, that is a popular size for A.net and other popular photo sites.
If you are using a different program, the menu items may be different, but the concept is the same. And remember - when you have resized your image, use "Save As" and CHANGE THE FILE NAME - that way, your original file remains unaltered!
KC-135 - Passing gas and taking names!