A family of Microsoft word processing software products for creating web, email, and print documents.
The approach depends on the kind of dropdown you're using; presumably formfields as you're using a Mac. In that case, for a given colour you could use a field coded along the lines of:
{IF{REF DropDown1}= "Red" "Red flower image"}
where 'DropDown1' is the dropdown formfield's internal bookmark name.
Note1: The field brace pairs (i.e. '{ }') for the above example are all created in the document itself, via Ctrl-F9 (Cmd-F9 on a Mac); you can't simply type them or copy & paste them from this message. Nor is it practical to add them via any of the standard Word dialogues. The spaces represented in the field constructions are all required.
Note2: For the process to work, you'll need to check the dropdown's 'calculate on exit' property and apply 'filling in forms' protection to the document.