i have some experience of this.
first of all, it would be handy to know which graphics software you're using. Photoshop is the business, as anyone will tell you.
images used in briefings get distorted, as you pointed out. you may have noticed elsewhere that the original files need to be of certain dimensions - using power of 2, i.e. 128,256 etc. - but in the briefing, these dimensions can be stretched to whatever you like.
the answer lies in creating your image at the size you intend to display it, which is where photoshop comes in.
let's say you have a picture that's 456 x 318 pixels. not powers of 2 (as far as i know - don't look at me for maths help, i'll be under the table shivering) so the image won't be recognised by ofp at that size.
so what you do is stretch the picture within photoshop, using the image size command (unchecking the 'constrain proportions' box) and make it, ooh let's say 512 x 512. now it looks bizarre huh? bizarre, but perfectly acceptable to flashpoint.
then in your briefing html, use the html format kindly supplied by THobson above, and make your dimensions 456 x 318. presto. flashpoint will helpfully un-distort your image.
hopefully
if you need a hand, attach the image to your next post or provide a link, and i'll be happy to fix it for you. it's m'job after all.