In spite of recent improvements in non-visual web access, images on the web still present an accessibility barrier to screen reader users. For this population group the presence of inappropriate text alternatives for images, or simply their absence, usually results in a poor web user experience. In this paper, we propose a controlled language (CL) rule-based approach that enables translation professionals to ensure image accessibility during the web localisation process. We describe the set of 40 CL rules we developed and then present the results of the evaluation of a selection of ten rules from the set. During the study, which sought to assess their impact on the appropriateness of text alternatives in French, the ten rules were applied using Acrolinx, a state-of-the-art CL checker. The results of the evaluation suggest that this sub-set of ten rules can help translators significantly improve the level of image accessibility obtained in the localised web product.