If you're using any kind of photo gallery script (Coppermine, Gallery, etc...), then yes most likely it comes with the ability to resize your images after you've uploaded them through the script- BUT, this feature can create resource problems on the server and bog down your hosting account. (With the exception of PhotoCart from PicturesPro.com if you're using their Uploader software - their software is excellent at auto resizing your images before the actual upload takes place).
For example - let's say you have photos that you took with your shiney new bazillion megapixel digital camera. Chances are they're at least a couple thousand pixels wide or high, and probably anywhere from 600kb to 3mb each. If you upload those pics and force your photo script on the server to resize them, rather than resizing them before you upload them, you can put a MASSIVE strain on the server and cause serious load issues.
Simply put; if you uploading large filesize images through your photo script on the server, you're forcing the server (which is likely serving thousands of people at once) to do all your image processing. If you consider how much of a toll that kind of process would take on your own computer with just you alone using it and several programs open, you can begin to imagine what it's doing to a server that's handling a lot of traffic. Not only that, but uploading massive filesize images will take you a lot longer than if you resized them yourself first, and your session could time-out.
FORTUNATELY there are plenty of FREE programs out there that let you resize your photos right on your computer BEFORE you attempt to upload them.
A good size for web photos is no taller or wider than 800 pixels, and that software will let you quickly resize an entire folder of images to 800 all at once with just a few clicks! (And of course, it gives you the option to put the resized pics in a new folder so that you still have your full-size pics on your PC as well).