I've seen something like this before, and it has nothing to do with Marketo. Are you by any chance working in a corporate environment where there are very strict internet controls in place? Try looking at the images outside your work network and seeing if they render, or using a different browser? There is nothing inside Marketo that is designed to prevent images from rendering, but often corporate security set to paranoid level can impact the images rendering in browsers. I hit this quite often in the financial services industry.
