Most people upload their pictures directly from their dashboard when adding a new post or page BUT, you can also upload and edit your images AFTER they’ve been uploaded by logging into your dashboard and selecting MEDIA >> LIBRARY.
From there you can view each individual picture and edit to suit your needs. See image below.

The default size for all images uploaded in wordpress are –
150 X 150 thumbnail,
300 X 300 medium and
1024 X 1024 maximum.
When constructing a new site, I usually change these to suit the page width for each website theme I’ve used. For example, the maximum embed size on this page is 520 pixels.
To change these sizes to the ones which are appropriate for your site, simply log into your website dashboard and navigate to SETTINGS >> MEDIA. See image below.
