The image doesn’t have to be directly representative of the memory. For example, my knowledge of the genetic causes of Down’s Syndrome is tied to the image of a tattered paperback copy of The Panda’s Thumb lying on a table in my stillroom. (This is an association that happened without any deliberate effort on my part. My memory does odd things sometimes.)
The important factors are for the memory objects to be distinct and precisely placed. In fact, things that seem out of place are often the easiest to remember. Puns and rebuses can also be useful in constructing a memory object.
Take your “myeloma” example. You could just make a skeleton with lesions and stand it in a room, but that wouldn’t be very memorable, especially since you’d have a whole bunch of other, similar-looking skeletons in there. Instead, picture a large, relatively clean environment, preferably one that has little to do with the subject. Say, a ballroom. On the floor, picture a lower-case “e” and “a” formed by a pile of dark earth. It’s your “e-loam-a”, i.e. “my e-loam-a”. Alternatively, the “e” could be formed of bones showing the characteristic lesions, so the image includes at least one symptom. Visualize that, “touch” it in your imagination, and repeat all the things you need to remember about it. Then move on to the next image.
A much less pleasant alternative, but one which also takes advantage of human memory quirks, might be to find names and pictures of people who have suffered/died with each disease. Picture them standing around the ballroom, grouped by related illnesses (e.g. all the cancer sufferers are standing by the door). Concentrate on their faces; humans are good at remembering faces. As you visualize each person, recite the list of symptoms they would have suffered, and whatever else you need to remember about the disease. Personalize the information.