Here’s a screencast I’ve made about writing pages, it illustrates how to add a single picture to your posts or pages: https://wphosting.tv/2011/11/workshop-writing-pages/
To add an image, use the little picture button next to Upload/Insert (underneath the post title):
You then have the option to add an image from your computer or from a URL (that’s how you can use images from Flickr). If you upload more than one picture from your hard drive, they can be displayed as a gallery – so your viewers will see thumbnails and can click on them to make them bigger.
To do this, first upload your pictures then hit Save Changes. Then once again click the Add Image button, but this time select Gallery from the top. Click Insert Gallery and something like this will be displayed:
When you save your post and view it you’ll see the thumbnail gallery.