How to "resize" photos\images before uploading to WordPress.

There is no way to "resize" the photos within WordPress; so if you have images of varying sizes and aspect ratios and you want your images to display uniformly...

Then, it will be necessary to edit your images and save them to a common size and aspect ratio before uploading them to WordPress.

Note: WordPress will create and use thumbnails of uploaded images automatically so the size and aspect ratio of the original image you upload is the key to properly displaying your images on the website. Its best to "resize" and test one image first (note the size and aspect ratios you use each time) Then upload it and see how it looks on your website.
Use any 3rd party graphic editing program like Adobe Photoshop, paint.net or GIMP to change the size of your photos before uploading the image to WordPress.

Any preference works but if asked we do recommend Paint.net (as its free, intuitive, and very capable)

Paint.NET can be downloaded here:
http://www.getpaint.net/index.html

Here is a video tutorial on cropping and resizing images:
https://www.youtube.com/watch?v=XRUSZqFYuRk