From Joomla! Documentation
These are the methods available for uploading images to your Joomla! website.
Upload Using Media Manager
The simplest way to add images is to upload them from your computer using the Media Manager. First, of course, you have to download the image onto your computer and be able to find it. Then, from the Control Panel (back-end administrative interface) navigate to Content → Media Manager.
On the left is a directory tree, with the root directory "Media". This corresponds to the default "images" directory, yoursite/joomla/images. Pick a subdirectory where you want the image located, or else do nothing to upload the image to the default images directory.
At the top left of the page click on "Upload". Then, you'll see Upload box. Click "Browse" to locate the image on your computer, then "Start Upload" to upload the file to the server.
Of course, you can upload images to a server using any standard FTP client. You might find this handier for adding images to template directories; however, if you have FTP set up, you probably don't need an explanation of how to add an image. Also, many server administration panels such as Cpanel and Plesk have upload capabilities.
There are several extensions available from the Joomla Extensions Directory which can upload images.