How to insert images in WordPress posts
You can insert images in the posts and static pages of your Wordpress blog from administration panel.
Log into your dashboard and click on the Posts menu on the left. If you want to add a picture to an existing post click on Edit under the post's name; if you want to create a new post with a picture click on Add New on the left. On the Edit Post/Add New Post page there's an Add Media button above the text editor. You can use it to add images (and other media files) to the content of the post. When you click on it a new window will open. It will automatically show the files in your media library (if you have any). To add an exiting image just click on it and then click on the Insert into post button.
If there are no images or you want to add a new one, you have two options. You can either upload an image(s) from your local computer, or you can use an image directly from an external URL address (hotlinking). To upload an image from your local computer click on the Upload Files button at the top of the window, then click on the Select Files button and use the small window that pops out to select and upload an image(s) from your computer. Then just select the uploaded image and click on the Insert into post button. When you mark an image (before you insert it) some settings will appear on the right. They are optional. You can use them to specify a title, caption and alternative text for the image; you can also change its alignment and make it a link.
If you want to insert a picture from a URL address, click on the Insert from URL button that's on the left side of the window for inserting media files. Then just type the URL in the respective field. It's not recommended to use hotlinking to display images in your posts/static pages.
In addition to inserting images in the content of posts you can also set a featured image. It's displayed above the post. Whether you can use this option depends on whether your theme supports it. If that's the case, on the add/edit post form you'll see a panel Featured Image which you can use to set a featured image.
For screenshots and some more details check out the tutorial on how to insert media files in posts and static pages in WordPress.
You may also find useful the tutorials: