Well ...
The bitmaps are a bunch of pixels. ArchiCAD or anyCAD works with vectors. Any automatic translation from pixel to vector is approximate. How can you expect to snap to pixels? Yep, it would be nice, I agree ... for all the placed bitmaps and PDFs. Just not very probable.
If you have a raster to vector converter already, ZWCAD or whatever, convert the image you have to a vector format readable by ArchiCAD, then open in ArchiCAD and refine. There is no way that you will get the required accuracy using any of the automatic converters out there - including the deceased Plan2Model, that actually worked, to an acceptable level.
It is often faster to trace, knowing the main dimensions, than it is to convert automatically and spend time cleaning up. Just think of any curved edge that will be gazillion of linear segments instead of a curve - that you define with two clicks.
Djordje
ArchiCAD since 4.55 ... 1995
HP Omen