If the PROJECT_LOGO is defined without a relative path, 
    PROJECT_LOGO           = LTXClogo.gif
the build complains that it cannot find the file, even though it is in an IMAGE_PATH directory. If the PROJECT_LOGO is defined with a path relative to the directory the build is executed from, 
    PROJECT_LOGO           = api_doc/img/LTXClogo.gif
the file is found OK and copied to the html output directory. However, the index.html cannot find it because the image source is defined with $projectlogo including the relative path:
    src="api_doc/img/LTXClogo.gif"

I can work around this by creating a link to the PROJECT_LOGO file in the directory I build from, then define it without any path. I think this cumbersome, 
the PROJECT_LOGO should be found and copied from the IMAGE_PATH directory, or the relative path should be removed when assigning the image "src".