This paper presents an architectural-space exploration methodology for designing approximate multipliers. Unlike state-of-the-art, our methodology generates various design points by adapting three key parameters: (1) different types of elementary approximate multiply modules, (2) different types of elementary adder modules for summing the partial products, and (3) selection of bits for approximation in a wide-bit multiplier design. Generation and exploration of such a design space enables a wide-range of multipliers with varying approximation levels, each exhibiting distinct area, power, and output quality, and thereby facilitates approximate computing at higher abstraction levels. We synthesized our designs using Synopsys Design Compiler with a TSMC 45nm technology library and verified using ModelSim gatelevel simulations. Power and quality evaluations for various designs are performed using PrimeTime and behavioral models, respectively. The selected designs are then deployed in a JPEG application. For reproducibility and to facilitate further research and development at higher abstraction layers, we have released the RTL and behavioral models of these approximate multipliers and adders as an open-source library at https://sourceforge.net/projects/lpaclib/.