AppImage is a self-contained executable for Linux (like a portable Windows .exe).

How to Install

First, download the .appimage from a trusted source.

Enable execution on the .appimage:

Alternatively, this can be done in the terminal:

chmod a+x <file>.appimage

You can now run the app by double clicking from the file explorer.

Alternatively, you can run from the terminal:

.\<file>.appimage

.desktop

Although optional, a nice way to pretty up and re-use .appimage(s) are .desktop files.

If not already, show hidden folders:

First, move your .appimage to a hidden folder ~/.appimage/ :
        NOTE: Make the folder if not already made.

Next, make a <appimage_name>.desktop in the your ~/.local/share/application folder:

(Notice how some appimages already installed themselves here)

Next, edit in a text editor our new <appimage_name>.desktop file:

You'll want the following at minimum:

[Desktop Entry]
Name=<YOUR_APP_NAME>
Exec=/home/<YOUR_USERNAME>/.appimage/<YOUR_APP>.AppImage %f
Type=Application

You can add additional fields, like tooltips and, icons, and MIME types:

Name=Publii
GenericName=Web Publishing Tool
Comment=Web Publishing Tool
Exec=/home/comfy/.appimage/Publii.AppImage %f
Icon=publii
Type=Application

Icons are sourced from ~/.local/share/icons using .png and .ico files:

Will now appear as launchable:

Read More

Adding to desktop: source
More about .desktop files: source
More about all keys for .desktop files: source