The icons parameter in the manifest.json file has a format of Javascript object, which defines locations of icons of various sizes. Extension should have icons of different sizes: 16x16 px, 32x32 px, 48x48 px and 128x128 px. Icons are ".PNG" images that should be placed anywhere in the extension folder. Image files can have any names. I have created 4 icons of appropriate sizes in 16.png, 32.png, 48.png, and 128.png files and put them into the icons folder inside the extension root folder. Then, manifest.json should be pointed to these locations using the icons parameter in a way, as shown below: