i want to prevent resizing my forum from bigger images.
Is there any way to do it like in this case (like , automatic resizing bigger images than XXXXxXXX resolution):

You can add this code snippet to your CSS Stylesheet (at the very bottom). This should resize all images within posts that are larger than 600 pixels (in this example, change the value if needed) to that value.

Note that this is not a thumbnail - they will appear smaller, but they still have their full filesize. Right-click and "View image" would display the whole, original image. The proportions should remain though, unless you defined an explicit width/height to an image.

Code:

.post img{max-width:600px;}

can you tell me please , where can i find the CSS Stylesheet in the Admin Panel?

admin, displays, colors, than youll css style sheet at the top next to colors in blue, click that and add the code above, and change it like said above, i use this and it has worked for me pretty well.

Thanx! Works Perfect