This has come up before with using the NPC images as the battle challenger images, not all of them end up being the correct size, which for the challengers is 300x300. I think some of them were fixed already, but here are some more that are not currently the size that they should be.

Johannes - https://img.subeta.net/battle/opponents/johannes.png 309x300

Kain - https://img.subeta.net/battle/opponents/kain.gif 268x300

Fantine - https://img.subeta.net/battle/opponents/npc_fantine.png 237x300

Lindsay - https://img.subeta.net/battle/opponents/lindsay.png 224x300

Carl - https://img.subeta.net/battle/opponents/npc_drunkcarl.gif 420x300

Euclid - https://img.subeta.net/battle/opponents/npc_euclid.png 215x300

Aspen - https://img.subeta.net/battle/opponents/aspen.png 253x300