If you want to upload an image for a future post, from the dashboard go to Media –> Add New. From here you can bulk upload multiple images before you know which post you want them to go into.
If you know what an FTP program is, you can upload images directly to the /wp-content/uploads folder.
For more information, see FTP Client.
WordPress can resize images and create thumbnails.
Refer to the following for more information:
|Theme featuring Image in Header.|
You can add images to your WordPress site through the use of template tags, Plugins, and in the stylesheet of your Theme as background images. It is really up to your imagination and web design skills on how you include images in WordPress.