FancyMenu's resource system allows you to use resources from Minecraft's own resource loader (Resource Packs), Local resources (files from the client system) and Web sources (files stored online).
Minecraft uses so called "resource locations" to "point" to a resource.
Resource locations written as text consist of two parts, separated by colon (:).
The first part is the namespace and the second path is the rest of the path to the resource, including the resource name with file extension.
The namespace of a resource location is always just the top-level directory/folder of the full path to the resource.
So lets say you load a resource pack with a resource called image.png that is stored in /assets/custom_resources/images/image.png.
In this case, the namespace of the resource location would be custom_resources, because /assets/ is just where Minecraft loads all its resources from, so custom_resources is the top-level directory of the resource.
This means that images/image.png is the rest of the path to the resource.
So the correct resource location to the image.png resource would be:
custom_resources:images/image.png
Fun fact: Since Minecraft has most of its resources stored in
/assets/minecraft/, the namespace of most Minecraft resources isminecraft.
The easiest way to load resources is to simply use local files stored on the client (and in most cases shipped with modpacks).
FancyMenu only allows loading local resources stored in /config/fancymenu/assets/, so make sure to store all your resources there!
This also makes it really easy to ship local resource with your modpacks, since most modpack systems (CurseForge, Modrinth, etc.) support shipping mod config folders by default.
When you need to dynamically change resources without the need to update your modpack, web resources would be the best option.
A web resource is basically just the URL to a file stored on a server, so let's say https://example-domain.net/image.png.
Make sure to always use DIRECT URLs, which means URLs that end with the resource's file name and extension, just like the example URL above.
Using non-direct URLs hurts performance and is more likely to fail.