Loki by Arthur Rackham.
Loki, Alberich and Odin by Arthur Rackham
Loki, himself a shape-shifter, pretends to be alarmed by Alberich’s ability to take the form of a serpent. He pretends to doubt Alberich’s ability to shift into smaller forms, at which Alberich transforms into a frog. Odin and Loki then trapped the frog and stole his magical helmet.