It is well-known, and mentioned in the GIMP documentation, that GIMP fails to handle PNGs with specified offsets. (However, I could not find any bugzilla entry about this.)

Utilities like IM import produce PNG images with oFFs (and vpAg) chunks,
so that a screenshot of part of the screen comes with the added information
about how large the entire screen is, and where on the screen the image rectangle is found.

It would be easy, and compatible with the spec, for GIMP to just ignore the PNG oFFs (and vpAg) chunks. It would also be reasonable if GIMP created a transparent layer with a size given by the vpAg chunk, and positioned the image in that layer
with an offset from the top left corner as given by the oFFs chunk.

What happens instead is that GIMP loads a PNG of size hor x vert and with offsets hoff,voff into a rectangle of size hor x vert starting at coord hoff,voff so that the image is partly or entirely outside outside the layer. Now GIMP shows a popup
"The PNG file specifies an offset that caused the layer to be positioned outside the image."

This bug means that GIMP cannot handle PNG images as produced by ImageMagick import (or convert, using the X: source).

See also http://www.gimpusers.com/forums/gimp-developer/10668-proposed-usabillity-enhancement-for-png-handling
where Sven Neumann rightly concludes that the current GIMP handling of these offsets is totally incorrect.