You can enter images into pages & posts (standing alone) or as galleries – galleries will tend to work better if you have more than 1 image as the re-formatting of the screen for mobile devices can get complicated (and not something you’d want to try to perform in your page/post!)
You’ll find it very useful to set a “featured image” on each page/post that you create. Featured iages are set in the bottom Right hand corner of the “edit” page. This image is the one that will be shown in summaries and lists of pages (the front page images are “featured” images.
Featured images should be 1000x600px in order to be big enough to not interfere with the formatting when changing screen sizes / devices. In order keep the site looking tidy – they should always fit into this ratio (5:3).