Actually, the imagesdir: property is not used/set at all. Missed it in the impl.. but, for the html backend, the image dir is only prefixed on the image:. macro to create <img src="#{imagesdir}/image-name" /> ?

So if anything at all, it should be the 'remote' url. 

using image:/images/blog/x.jpg would do the same as setting the imagedir= /images with image::blog/x.jpg ?