Having wrestled with this subject for many months a year ago I'm 100% confident that the best advice anyone could give you is to start by upgrading to 6.5. If you can't do that you're opening a can of worms, because version 6 has a bug in handling pixel aspect ratio which was never fixed in that version. Jeremymoore.com has a section describing it and the fix for it, but it's a lot of work and if it isn't your only option don't go there. I did work my way through it, including installing his fix but I wouldn't wish it on anyone. Not to say I didn't appreciate Jeremy's pioneering, I very much did but I'm SO glad 6.5 got it right.
Having said that, to begin to make it right without the fix you need an image editor capable of creating a blank DV frame sized image (720X480) and allowing you to drag a picture into it and resize it with a tool like free transform (Photoshop for example, maybe PaintShopPro too). If the image you're working with isn't in the 4:3 aspect ratio something will be lost in stretching it to fill the frame - top-bottom or sides. It also has to have enough native resolution to be stretched without breaking up. If it is larger than DV you can shrink it without loss but you can't make it bigger without degrading it some. If you have this capability and want to pursue the solution, post again and I'll go into more detail.
David Hurdon
http://www.contentshop.tv