I have to think about the skin. The zimreader needs a user interface and this is realized as html files. This is what is called "skin". I currently don't exactly know, how to change that. I need some dynamic html in the application.

The content-type is perfectly ok in zimreader. Your zim file is wrong. And I know also why. The mimetype column in article is now a text instead of a integer. You have to put the actual mime type into that column. Remeber, that we do not use a fixed enum for mime types any more but a dynamically built list inside the zim file. This is also the explanation, why your images are slower. They are indeed clustered and compressed, since the mime type do not match the hard coded mime type list found in zimwriter/src/zimcreator.cpp (image/jpeg, image/png, image/tiff, image/gif, application/zip). It is normally not a good idea to have it hard coded but I currently have no better idea.