Similarly to posting audio, when you edit or create a new node you will see an area above the body of your post, made for uploading a single image. If you want to upload multiple images to the same post, there's an option below the body of the post for making a photo gallery.
Clicking in the white box of either the gallery or the single image uploader will bring up your computer's finder. You can only add one photo if you are using the single photo uploader. If you use the gallery, you have the option to keep adding new pictures (one at a time). Multiple images will be added in a scrolling gallery.
After selecting your images, make sure that you click the blue "upload" button, and then save your post.