How to add an image to an article
From the editor
To add an image to an article first open the article for editing. (Joomla->Content->Article Manager and click on the article you want to edit.)
Click the image tab at the bottom of the page. This will open an inset page showing you the available images. You can click on one of the images, in which case the Image URL will show in the form. You can also assign an Image Description (used if the image isn't available) and or Title (shows on mouseover) here. You can also set the alignment using the drop down list on the right.
If you want to add an image from you computer, click on the 'Browse' button at the bottom of the inset page. This will let you browse your computer to select an image file. Once you have selected the file, you can click the 'Start Upload' button to add it to the images available to your stories.
When you have an image selected, you can click the 'Insert' button on the top right of the inset screen to place the image in your page.
Also, once the image is in your page, you can click it to select it, then click the icon with a picture of a tree in the tool bar to access the styling controls for the image. Be sure to look at the "Advanced" tab, which lets you control the displayed image size, the border, the margins, and the float. If you don't see the css style element you need you can add it in the text box on the "Advanced" tab or in the html. (You access the html by clicking the "toggle editor" button at the bottom of the page.)
You should also take a look at the Media Manager at Joomla->Content->Media Manager.
Here you can see and manage all of the images used in your site. If you have a lot of images to upload, this is a convenient way to do it.
Another way to upload images is using an FTP client. The images are all under the /images folder in the root directory of your site. So, for example, the images that appear in articles are in /images/stories.
Be aware that file names in Linux are case sensitive, unlike Windows. If your link refers to 'MyPicture' and you upload it as 'mypicture', it won't work. This is usually the problem when an image goes missing.