Yes, you are right. I figured that out after I put that comment there. Image width and window width are precisely the same,so the truncation happens for a different reason.
On Fri, Jul 19 2013, Wolfgang Jenkner wrote:
>> + ;; on GNU Emacs 24.3.50.4 (i686-pc-linux-gnu, X toolkit, Xaw
>> + ;; scroll bars) of 2013-07-16, image width is slightly
>> + ;; truncated, ~6px,
Perhaps, the (apparent) truncation you see is the result of not using
a right fringe, in which case there's now always some space on the right
of the window used for continuation or truncation glyphs, even on
graphical displays, even in image-mode, see bug#11832.
Wolfgang