The Legend graphics URL of WMS layer can be found at the setting of WMS layer at Geoserver
For example, here is a layer setting at layers.js in GLTG
In geoserver, get in "layers" setting (you can find it in the left side menu as shown below.
Find the layer with id, "gltg:gltg-pools", and click the the title of the layer.
The style setting is under "publishing" tab as shown below:
You can see a graphic image under "default style: gltg:pool". Right click on the image and copy the URL (in chrome, "copy image address").
The URL of the legend for the lyaer is as below:
If the legend graphics is needed to be modified, you need to edit a style called, "gltg:pool".