Book Designer manages to centre it but of course it can't do in-line images as your libprs500 can.
Here's an output from Book Designer's LRS with an image that is 550x724 in size (it is centred):
Code:
<ImageBlock x0="0" y0="0" x1="524" y1="690" xsize="524" ysize="690" blockwidth="524" blockheight="690" refstream="im_1" objid="pic_1" topskip="0" sidemargin="0" blockstyle="bs_im">
</ImageBlock>
<BlockStyle stylelabel="bs_im" objid="bs_im" blockwidth="524" blockheight="704" sidemargin="0" blockrule="block-fixed" layout="LrTb" topskip="0" footskip="0" framecolor="0x00000000" framemode="square" framewidth="0" bgimagemode="fix"/>
If you want more information let me know and I'll log a bug. I can of course send you the files you need.