Welcome to the forum, vadude. My license doesn't include MeshCAM Art, but I do machine bitmaps with regular MeshCAM.
If you load "fuzzydots.bmp" for instance with "Black is Z+", the machining origin is at the bottom of the geometry, which is what MeshCAM considers the geometry zero. If you go into CAM | Set Program Zero you can set the vertical offset to the top of the geometry. That will make all the machining Z coordinates negative.
If it is something other than this, we'll still sort it out together.
Randy