I saw a couple of forum questions about how to add a logo to an AL Extension and I thought to share a quick guide on how to. 
 
If you publish your new extension to Microsoft Dynamics 365 Business Central and then navigate to Extension management page, you will see your new extension as below (My extension is "Demo Extension"). 


You only need to follow a couple of simple steps to add a logo to your new extension.
1. Place the image into your Repo.
2. Refer to the image from app.json file
You can simply copy the image to any folder that you have within your workplace. I always prefer to keep things organize so I know where my .al files and other extension related files are. Therefore I will copy the logo to my res (resource) folder where I keep all my resources related to the current extension. 

Once the above step is complete, you can open the app.json file and set the logo setting to refer to the logo you just copied. Please refer below screenshot :

You might have noticed that my file path starting with ".", it means the current working folder or in my case the Repo.

Once you have the "logo" setting set in the app.json file you can simply compile and deploy the extension to Microsoft Dynamics 365 Business Central. This time it will get published with the logo.  

 

Please provide your feedback with a comment. 
Thank you and Regards,
Tharanga Chandrasekara