I have very basic knowledge of php and SQL, but basically you want to read the data, and the new piece onto the end, and then re-instert it... but you already knew that. Hope someone else can help more than me
Or what you COULD do, is have a field for every item in the member table, and when they buy it change it from "no" to "yes", and then when displaying the image, you could just say list from XXX fields where item = yes. Just an idea.