Who knew that there are 5 different ways to change your background image in Ubuntu?
Method 1: Use Settings to Select a Default Image
This method is one that most people find out right away. To do this, right-click on your desktop, and click “Change background…”

It will bring you to the Settings app. You can now select a default Ubuntu image.

Method 2: Use Settings to Select Any Image You Want
Once again, right-click on your desktop and choose “Change Background…”

Like last time, it will bring you to the settings app. This time, click the “Add Picture…” button at the top.

Choose your file.

Method 3: Use FIrefox
Open Firefox and search for an image. Right-click on the image, and then choose “Set as desktop background.”

Unfortunately, if you search for an image and right-click on it, it isn’t usually a full-quality image. If the website that the image comes from has a download button, use that button and use the next method instead….
Method 4: Download an IMage and use the File Manager
If you find a website that has a full-quality image, download it and open the file manager.
Find your file, right-click on it, and choose “Set as wallpaper.”

When you click “Set as wallpaper,” the image will most likely be saved in ~/Pictures/Wallpapers. This is great because you are now allowed to delete the image from your downloads but still keep the wallpaper. If you delete the image from ~/Pictures/Wallpapers, your desktop image will go blank.
Method 5: Use Terminal
In this method, you will use Terminal to change your wallpaper.
Run this command:
gsettings set org.gnome.desktop.background picture-uri file:////your/file/path/here
Of course, instead of /your/file/path/here, you’d put your own image file path, like:
gsettings set org.gnome.desktop.background picture-uri file:////home/alex/Downloads/cool-image.png
I find that this method is a bit more of a hassle, but there might be a time when you’d need it.
Conclusion
I created a video on how to do this. Here it is:
If you have any questions or comments, please put them in the comments section below. Thanks!