Yep, I am trying to do the same thing, any png file is not recognized by the library browser that I put in the library and try to insert it. What format is Qcad using in its library items? For example the templates are in 64x64 png and only ~500 bits. But if I try to export a 'block', it is png 5,ooo bits big. And the my 64x64 version is unrecognizable.
Trying to copy a seperate dxf file and paste on to a new layer on my drawing does not do anything for me.
I am using linux.