this is typically resulting from a template (or email if it's created outside of Marketo) not correctly developped (sorry for being blunt here, I do not want to hurt anyone . In fact, developing an email template that will render on any platform is really a difficult task). Nothing we can do here excepted to advise you to look into the services of a well performing template service (knak.io) or skilled template development firm (Grazitti, Hoosh, Perkuto, ...)
If you do not have access to a developer to adjust your template and make it mobile friendly. One trick might work is to set your image size to width="95%" height="95%". That way image will adjust its size to 95% of it is allowed instead of a fixed dimension. Hope it'll work for you.