Is it possible to use an Image Texture inside Geometry Nodes as a texture for Materials?
That'd be possible, I think what you're currently missing with your setup is the Set Material (and a Subdivide Mesh) node.
Also, an additional reminder. When your attribute is being outputted, it's nameless (or basically anonymous), which means that you can't tell Blender which attributes you'd want to be used. So, for an attribute to be usable in the Material Editor, you'd want to give the attribute a specified name (by typing in the blank space in the Output Attributes section in the Geometry Node modifier).
Once you're done with that, you can use that attribute in Material Editor.
The result:
The node setup:
