Hi ysc,
I found a solution https://community.openhab.org/t/display-local-images/2592/9
Copy the files to conf/icons/classic and address them via the following URL:
url="http://serverip:8080/icon/filename.png"
Works fine for me. Important the spelling of the url path. “icon” is correct although you store it on /icons/classic on the filesystem.
Best regards.