Slidebean gives you a free integrated gallery of images, icons, and gif animations. Go ahead and take advantage, there are tons of them. Of course, you can also upload your own images.
Just hit Add Content Block to insert the element on the slide and select whether you want an image, icon or gif. Click on the added block to select an image, a window will pop up for you to browse using keywords.
For images, there are two galleries: Unsplash and Flickr. Feel free to browse and figure which one has the images that suit you better. Giphy is all about gifs, if you're looking to give the slide a fresh, funny touch, and finally Noun Project will give you tons of cool icons.
Once you add one of these, in the Design editor you can modify its Size, Caption, and Cropping style as well as replacing the image. Just click on it and a panel with the editing options will show up. You'll also find the Delete option at the bottom.
To have a Background image, go to the Design editor and find the Settings button at the side of the slide. A panel with options for the background will pop up and you can click to choose an image. You may also use Icons and Gifs as backgrounds 😉
Once added, you can Dim it for better contrast.