The best way I can think of is to keep the two images in BO server and refer those in a free standing cell within WebI using boimg://<image_name>. You can toggle the images using if-else logic.
Path to store the images:
<BO Installation Directory>\SAP BusinessObjects Enterprise XI 4.0\images
Example:
You need to set the property of the cell to read content as image hyperlink.
In the above example, the logo is changed based on input control selection. To change the logo based on prompt selection, create a variable to capture the prompt selection using UserResponse function (for example, v_Selection=UserResponse("Prompt Text")). Then use the variable in if-else logic to toggle the logo.