Windows10 includes "Paint", which can resize photos/images.
(it's what i use 98% of the time, and i've posted hundreds of images to the forum)
If you right-click on a .jpg file, you get "Open with..." as a menu choice.
Mouse over that and Paint may/should be one of the choices.
Once *in* Paint, there's a Resize choice on the top menu bar, and it will display/perform based upon percentages or pixels, your choice.
Adjusting the width automatically adjusts the vertical to maintain the aspect ratio (unless you untick the choice box).
(example produced with Paint)
Another free program is Google's Picasa, which also includes a handy image viewer
In the Picasa editor, there's an "Export" choice on the bottom right of the screen... that's the easiest way.
Picasa produces smaller file sizes than Paint.
--dick