> I don't see an option to change this in the Restrictions
> section of my teacher account. ...
> Is the limit for images unchangeable?
If you store your project in the cloud, I apply limits on the amount of data you can add to the project (as a way of keeping my running costs under control).
If you store your project on your own computer, I have no need to apply any limits, so you can add as many training images as you like.
> As I write this out, I'm realizing it's probably good that it catches both...!
It's subjective - there's no "right" way to do this, and you could absolutely train a model to only recognise squares with the duck covering a majority of the space, or you could train a model to recognise squares with any sign of the duck in it. Both are valid, and potentially useful. But personally I tend to agree with where I think you ended up here, that a model that identifies all the squares the duck is touching is a good approach.
Kind regards
D