Rory,
I'm not a coder so can't help there. However, if you go into assignments there's a descriptions box after the title. I often put links to files, images there. It's as simple to use as any other dialogue box - click on 'insert image' and away you go.
If you have a large number of images e.g. Art, then you should be able to create a Gallery (Lightbox Gallery seems popular). Then, upload your images and put the link into your instructions. It's a hyperlink so no worries (set the display option to 'new page' in force download - makes it easier for students to follow numerous images rather than scrolling back.
Hope that helps.
Paul