Identify if any existing images/figures would benefit from custom illustrations

[arronlacey]

I have put placeholder images i.e. a ROC curve that has been sourced from a blog / site and maybe it would be better if a custom illustration might work better? We could produce these via an example analysis we conduct ourselves (and link a notebook) if that doesn't sound the way to go? Just thinking about consistency.

For example here is a placeholder image I am using in a PR here