The above picture is an example of the gallery area on the alertbox settings page. The gallery can be used to upload custom images and sounds to TwitchAlerts by pressing the square (as shown in the image)
However if you are running an incompatible or out of date browser you will see the following :
If you are already using one of these browsers than your browser probably needs to be updated. A good website to use is www.whatbrowser.org . This website will tell you what version you are running and provide instructions on updating the browser you are using.