To add a .jpg into the briefing, use something like the following line
<br><p align="center"><img src="picture.jpg" width="64" height="64"></p>
the actual jpg is stored in the mission folder
Dont bother using too high a resolution, this only adds to filesize, and once you get to a certain resolution, it doesnt appear any better. I forgot what the max resolution i used was
For size you can use a combination of the following width, height sizes
16 32,64,128, 256, 512
I find 256 X or greater is a bit too big for the page though
as far as adding it to the cutscene is concerned
here is an example of the Description.ext entry
class RscTitles
{
   titles[]={starttext1};
  Â
   class starttext1
   {
      idd=-1;
      movingEnable=0;
      duration=6;
      fadein=2;
      fadeout=2;
      name="starttext1";
      controls[]={"Screen","Titel1",};
     Â
      class Screen : RscStdText
      {
         style=48;
         text="MYPICTURE.JPG";
         x=0.2;
         y=0.2;
         w=0.6;
         h=0.1;
      };
and here is how it is called in a script
titlecut ["","black in",3]
TitleRsc ["starttext1","plain",8]