To use your own custom icon in the Minecraft client server list you must have a perfect 64x64 size .PNG format image named "server-icon.png" in the main server folder.
- Access your server files using the web based file manager (Files -> FTP File Access) or connect to the FTP server using a desktop FTP client.
- Find your "server-icon.png" on your PC and upload it to the main server folder, which will show as "/" in your FTP client, or the main folder that opens when you login to the web file manager.
- Upload it and make sure the image dimensions are exactly 64 x 64 in size. As well, the image must be PNG format and named "server-icon.png". If you have an existing .jpg file, you can't rename it to .png, you must use an image editor to convert to PNG format.
- Once uploaded, restart your server.
If you want a free, easy to use image editor to create your server icon image file or to convert an existing image to PNG format, we recommend using Photopea.
https://www.photopea.com/