either that or you reference another richtextlabel object, like richtextlabel.add_image(t,10,10)
As many other coding langues, in Godot all the functions (methods) are linked to an object (nodes): you can refer to any node (and call its related methods) from every piece of script, you just have to specify it before the ".
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If you dont specify it, Godot assumes you are referencing the same node the script is attached to