Images resize problem

Hello.
In admin panel I've setted
Images resize:
Width: 1100
Height: 900

it looks like this on my 1600x900 monitor:


When I click: Enlarge this image, then it's:


How can be "Enlarge this image" enabled default? My forum uses people with low screen resolution, so for them it looks like this:


I want to have max-width of images resized to 100% of current resolution, but without down scroll-bar.

When I tried with CSS:

Code:

.postbody img{
  max-width:80%;
  border:0px;
  width: expression(this.width > 600 ? 600: true);}

It worked, but:

Down scrool-bar was still for 1600x900 images

Please help

bump