(Tutorial from someone elts)
In other words, Its not my writings but it will help you.
"""Actually this is a very simple process.
OFP can use any JPG that has sidelength to the power of 2 (in Pixles), so basically all you have do is make sure you pictures have sidelengths of 2,4,8,16,32,64,128,256,512. Please note that OFP does not give all that much space in the Overview and Briefing to show a picture. In the Overview 64x128 works fine sizewise.
"What the heck, I wanna make a picture with 170x23 pixels"
No biggie, all you have to do is resize the picture the next higher allowed resolution, in this case 256x32.
Now the picture looks somewhat wierd. Well, but most of you should know that the Briefing and Overview are actually HTML files, and all you gotta do is put your original Width and Height values in there. And voila, it shows as it was suppost to in the Briefing.
A Mistake that seems to be often made is to put the picture with some sort of HTML-Editor in which also inserts the name. The line with the picture should look like this:
img src="mypic.jpg" width="128" height="64"
Make sure there is no URL or Pathname where the picture name should be, only the name of the picture.
A few notes on the side:
To make it look somewhat fitting with the already existing official Paa's, I usually create a small white border around my picture (I use PSP), and add a little noise before that. I also noticed that the size you give in the HTML files seem not to be the size it is actually shown. I usually go to a higher resolution and let it scale down by OFP via the HTML files because somehow it seems to be a better quality then."""