Hi
You only have to do this:
in \templates\default\media\
jpg.htmlyou must edit to this
<!-- Template file for JPG Files -->
<!-- <img src="{media_src}" border="1" alt="{image_name}"{width_height} /><br /> -->
<img src="{media_src}" border="1" alt="{image_name}" width="640" /><br />
I've remarked the original line with <!-- .......... -->
The width "640" or "800" works perfect and you dont have to say the height!
If you are uploading PNG or GIF in such dimensions you have to do the same mod for png.html and gif.html