The way I fixed this problem on my site is by editing the Template in the pm module ... pm_readpmsg.html

The line ...
<img src='uploads/<{$poster->getVar("user_avatar")}>' alt='' /><br />

Change to ....
<img src='<{$xoops_url}>/uploads/<{$poster->getVar("user_avatar")}>' alt='' /><br />

AND ... the line that begins with ...
<img src='images/subject/<{$message.msg_image}>' alt='' />&nbsp;

Change to ...
<img src='<{$xoops_url}>/images/subject/<{$message.msg_image}>' alt='' />&nbsp;

All I am really doing is adding the <{$xoops_url}>/ in front of each img src statement

Remember to do an "Update" to your module afterwards.

Worked for me anyways But I am sort of a newbie so ... ???

HTH
Peter