Click the Media Manager icon in the Control Panel, or click Content → Media Manager menu in the back-end of your Joomla! installation.
The Media Manager is a tool for uploading or deleting files in the <joomlaroot>/images/ directory on a web server. Tools included are: uploading new file(s), deleting existing file(s), and creating sub-directories.
At the top right you will see the toolbar:
The functions are:
To alternate views between thumbnails and details simply select the view you want by clicking "Thumbnails View" or "Details View".
If you would like to upload images or other files to the <joomlaroot>/images/ directory (or any of the sub directories) simply open the directory you want to upload in by clicking on it in the file navigation utility on the right side of the screen. Near the bottom of the Media Manager, there is an area marked Upload File [Max 10M]. Underneath that heading are three buttons: "Browse", "Start Upload", and "Clear Completed". To upload a file, start by using the "Browse" button to find the file you would like to upload, and then click "Open". (You can do this several times so long as the overall file size accumulation doesn't exceed your max file upload size). Once you have all the files you want queued, hit the "Start Upload" button. The files will be uploaded to the selected directory.
If you want to create a sub directory, you can do so using the Media Manager. Navigate to the directory you want the soon-to-be sub directory to be located in. In the upper right hand side of the Media Manager, there is a text field area next to a button labeled "Create Folder". Type the name of the desired sub directory into this text field, and then click the "Create Folder" button. The page will refresh and the sub folder will have been created.