QCAD estimates that the bitmap would take up a lot of memory and does not export it to avoid crashing due to lack of memory.
This behavior of QCAD can be changed only by editing the configuration file of QCAD directly.
You can edit the QCAD.ini file with a plain text editor of your choice.
After the section [Appearance], please paste the following line:
AutoHideImages = 0
Depending on your operating system, you can find the QCAD.ini file at:
C:\Documents and Settings\username\Application Data\RibbonSoft\QCAD.ini
Max OS X:
(username is your login name)