Hey Yousosmart, remember that my opinion is just that of one person! But I feel like you deserve a bit more of an explanation.
When I think of a thief the following visuals come to mind:
Inconspicuous: A thief who is trying to steal something will be doing everything they can to remain unnoticed/unrecognised - muted colours, cloak/hood, covered face, no bright eye-catching punch colours, soft shoes do muffle footsteps, no armour that would make sound when moving.
Occupation focused: A character that identifies as a thief (as opposed to someone who just happens to steals things) would have two primary modes; Planning and Executing. During planning a thief might look very different to a thief executing a theft but even then they would need visual indicators to show they're hatching a brilliant scheme. During a theft everything about them should show their intention - easily accessible tools for break in, a place to secret treasures once stolen, weapons or escape tools in case they get caught, items for distraction.
Necessity Driven: Whenever I think of thief characters from stories they are most often depicted and driven by necessity. If they could just barge in the front door and take what they need then they would just do that but for whatever reason they can't and so they must find another way. They are often stories about overcoming a more powerful force by using wit and skill. This practicality and utilitarianism often extends to their appearance.
Having said all that, if you're designing a champion then the artwork you're using is fine because it's just there to give people a launching point from which their imagination can build upon.
