categorymembers takes it's parameter for category without the namespace while imageusage takes with it. I suggest add the namespace name to categorymembers and rename it to cmtitle instead to match the other functions.
Version: unspecified
Severity: enhancement
To clarify what might seem unclear to some: list=imageusage currently takes its iutitle parameter as Image:Example.jpg, while list=categorymembers takes its cmcategory parameter as Living_people (as opposed to Category:Living_people). The suggestion is that cmcategory be renamed to cmtitle and require the Category: prefix.
I'll implement this when I get around to it.