Instead of inserting the image directly, you could Link to the image, if it
is not being distributed then this can work, again the user will need to be
instructed on how to do this.
Create a lower Resolution and Size version of the original image and insert
this instead, again the user...
I don't know of any way to do such as policy, but someone else may jump in
here with a macro but how this is distributed to the users, again the user...
;-)