What I've tried was to add a resource using reshacker:
Than I created aCONTROL "", 1999, STATIC, SS_BITMAP | WS_CHILD | WS_VISIBLE, 0, 0, 109, 222
and a matching function:Page custom FirstPagePre
the problem is that the default image is hiding the custom image:
Function FirstPagePre
GetDlgItem $0 $HWNDPARENT 1999
ShowWindow $0 ${SW_SHOW}
${NSD_SetImageOLE} $0 "http://localhost/sidebar.jpg" $1
FunctionEnd

I also tried using inetc to download the image (a bmp image) and to use the command SetBrandingImage but I got the same result.
Thanks,
J.