Illustration of the influence of complexity in the modelled source. Top: mock image created for 3000 s exposure time and a bright source. The lens has the same characteristics as for the base case (see Sect. 2.1), but is discy with a₄ = 0.04 in a fold configuration. Bottom: fit of the above image with (right) and without (left) adding shapelet reconstruction for the source during the fitting process. These results are also displayed in purple in Table 1.