It really depends what you are looking for, as an organisation that does a lot of dynamic web work involving images, I tend to like to ask a geometry question pertinent to the job. In any case, I tend to ask a geometry question, as I find it a good maths test that is nice and visual and can show a candidates ability to visually present their working and methodically work through a problem.
For advanced candidates, I occasionally give the following question:
This image shows a crescent moon. The width of the crescent from B to D is 9cm and between E and F, 5cm. C is the centre of the larger circle.
a) Please calculate the area of the crescent.
b) Describe the calculations necessary to resize an image to fit in to the inner circle from any given size, and position it within the circle if the centre point is known.
For an easier question I usually give the same sort of question, but use the "square within a circle within a sqaure"square" example. Though this is very easy, so I would expect perfect algebra on it.
Over and above that, I tend to ask them to knock up an algorithm for generating all combinations of a variable length data set.
