You can add images to your pages and posts in several different ways: they can be embedded in the text, or be displayed as slideshows, galleries, or lightboxes.
To embed an image in text, simply upload it directly into the text flow. Use the Image dialog in the toolbar to resize it or to make it clickable, and use the text alignment tools to make it fit the flow of your content - just like you would in any Office program.
How to make a slideshow
Name the images with the prefix slide_ [slide_mypicture.jpg], and upload them as attachments instead of injecting them into the text. They will generate a slideshow automatically. The order of the slides will be the order you uploaded them in, with the newest one first. The pictures on this page consists of two such slides.
How to make a Lightbox
Upload the images into the text area in a row [with no line breaks between them], then add links to each image, pointing to the image itself. The URL will always be pagename/imagename [you can use the Image Format dialog to shrink them to thumbnails]. When a non-logged-in visitor clicks any image that is linked up this way, it will open in a lightbox with navigation buttons for browsing the entire row. Click the slideshow above to see an example of how this works.
How to make a Gallery
Create a Gallery Layout from the File Manager Picker, and upload your images directly into it. You can also use a Photos Document to make a Gallery - the difference is that a Layout will format the image for you, will display Exif data and Sharing buttons, and, since the images are not embedded in a document, they will be sortable.
Take a look in the Sidebar to see an example of a Gallery Layout...