In this series of papers, we develop a formalism for constraining mass profiles in strong gravitational lenses with extended images, using fluxes in addition to positional information. We start in this paper with a circular power-law profile and show that the slope gamma is uniquely determined by only two observables: the flux ratio f(1)/f(2) and the image position ratio theta(1)/theta(2) of the two images. We derive an analytic expression relating these two observables to the slope, a result that does not depend on the Einstein angle or the structure or brightness of the source. We then find an expression for the uncertainty on the slope sigma(gamma) that depends only on the position ratio theta(1)/theta(2) and the total signal-to-noise ratio (S/N) in the images. For example, in a system with position ratio theta(1)/theta(2)= 0.5, S/N=100, and gamma= 2 we find that gamma is constrained to a precision of +/- 0.03. We then test these results against a series of mock observations. We invert the images and fit an 11-parameter model, including ellipticity and position angle for both lens and source and measure the uncertainty on gamma. We find agreement with the theoretical estimate for all mock observations. In future papers, we will examine the radial range of the galaxy over which the constraint on the slope applies, and extend the analysis to elliptical lenses.