Conditional image folders are not currently supported directly in Help+Manual, sorry. However, if you need this for WebHelp or CHM or EWriter you can achieve this with a skin with the help of the Toolbox Help+Manual power tools utility included in the Premium Pack, which allows you to enter alternative project search paths in the skin, which then overload the version in the project.
IMPORTANT: The skin must be stored in the same folder as the project you are publishing, otherwise it won't work! (The relative paths would then be wrong)
Suppose you have two image folders, img1 and img2, which should both be in the same parent folder. You would put default images in img1 and include only the img1 project search path in the project itself. Then you put the alternative images in img2, where they should have exactly the same names and dimensions as the versions in img1.
Then select the absolute project search path shown in the settings list and check the actual relative path stored, which will be shown below the box. For example, if the image folders are inside a folder called images, the relative path inside the project folder would look like this:
.\images\img1\
So then open the skin in Toolbox, select the Project Options button and add this to the Project Search Paths item:
.\images\img2\
Then save the skin with Toolbox. When you publish with that skin, the images will be sourced from img2 instead of img1.