If the image is part of a structured document, e.g., a DITA or an Author-it document, it is not searchable on its own via the Fluid Topics search engine. You can only access it when bringing up a document, because images are displayed within the context of document in which they are contained.
When hovering the mouse over the image in the reader, the pointer changes into a zoom-in
icon. By clicking on the image, the image file is opened in the Fluid Topics
Viewer page.