[Admins]

Both testimg and isonimg would be good I think. The function isonimg would be more efficient if you want to only check a few things and don't want to care about overlapping images.
The functions testimg, testnpc etc. should be changed to give the index of the first object found, and also fill an array with the indexes of all objects found on the given position, sorted by the drawing order.
(since the variables can be normal variables and arrays at the same time it might be useful, and it would be compatible with old scripts).