You can change the URL of the channel logos by clicking the logo icon in the editor next to the channel's name.
You change the logo in the m3u file, not in the xml file!
Many apps take logos from the XML file by default. If you don't see the new logo, change the settings in your app to get logos from m3u instead of xml!
You can change the logos only if you have set the option "use iptv-epg icons" in the editor. That's the default setting.