You just have to click on "media" button and it will ask you to upload a new image or to select one of your previously uploaded image. You can use the sidebar to choose a dimension. Tell us how it works. If we can help any how, just ask.
Hi,
It's very easy to add images in WordPress. When creating or editing WordPress Pages or blog post, you can add images by using WordPress Media Uploader Tool
Add media to your post or in your widged? If you want to add a media to your post: Dashboard => Media => Add New. And if you want to add media in widget, you've to install Jetpack