I'm going to explain you how to edit, add and delete the slideshow images in your site.
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 Joomla, navigate to Site -> Media Manager.
On the left there is a directory tree, with the root directory "Media". This corresponds to the default "images" directory. Pick a subdirectory where you want the image located (normally you have to pick the folder called "Slideshow"). You will see all the images used by the animation.
At the bottom of the page there is a typical Upload box. Click "Browse" to locate the image on your computer, then "Upload" to upload the file to the server.
* Please note that the images you upload have to have the same size as the slider doesn't resize your images.