Step 1: First of all you'll need an image with dimensions 64x64. This is the image size that will be displayed in your Minecraft Server List.
Step 2: Rename your file to
server-icon.png. Make sure it is a .png file and not anything else. Minecraft can only use .png images.
Step 3: Go to the DedicatedMC.io Panel and select the server of which you want to change the icon of.
Step 4: In the selected server, either go to 'File Management' or upload your files using SFTP. A guide on how to use SFTP can be found here.
Step 5.1: When using the File Manager on the Panel simply click 'Upload', select your server-icon and upload it to the server.
Step 5.2: When using SFTP simply double-click or drag your server-icon to the left column when using FileZilla. This will upload the file to your server.
Step 6: Restart your server. When you now refresh your Minecraft Server List your icon will appear on the left of your server!