Each slide item has an essential image upload option. To upload an image to each slide item, you have to select slide item for which you want to add the image.
When you click on a slider item, there are three types of options are displayed: Panel Title, Slide Content, and Background. By simply clicking on Background, you can change the slider image.
Once you select a slide item, you will find the image upload option in the right sidebar. Now, simply upload the image. It looks like the following screenshot:
