onClick javascript: navigation with changing css background images / two types

"Leistungen" has the class lmark, while "Impressum" has imark. Your Javascript function for lmark-classed elements resets the background for all other lmark-classed elements, but not for the imark-classed ones. Same for the function that concerns imark.

If you need to have only one non-default background for all lmark and imark elements, a correct (probably correct, though not tested) code is:

function li_mark(cell) {
    var elts=document.getElementById("up").getElementsByTagName("li"); //store the elements we need in a local variable so we don't have to rewrite all that line (and the browser won't have to do the fetching job again), we don't care if they're L-type or I-type
    for(var i in elts)
        if(elts[i].className && elts[i].className.length>=8) //check if there is a className and it is at least as long as "uplink-X", otherwise we don't process the element and skip to the next
            elts[i].className="uplink-"+elts[i].className.charAt(7); //we retrieve the letter for the type of class (L or I), and we append it to "upload-" to redefine the class of the current element
    cell.className+="-active"; //after the loop, all elements have their default class, so we just need to append "-active" to the one we want to be active
}

Then, all your calls to l_mark(this) and i_mark(this) should simply be replaced by calls to li_mark(this).

P.S.: not related, but my current browser (Firefox) doesn't seem to like the window.onload=setActive;, saying that "setActive is not defined"...

Edit according to comments, having seen your JS file (scripttests.js):

You still need 2 functions, mark() and li_mark(), because for mark() your active class is of the form <className>active instead of <className>-X-active (not the same pattern in naming the classes, and no need to retrieve the character Lor I).

Another part of the problem is that you call cell.className+="-active" twice on the same cell in the one function you have.

The last part of the problem is that you forgot the instruction within the if of the second loop: as there are no block brackets, the instruction that is executed when this if is true is cell.className+="-active"... executed as many times as the loop runs and the if is true.

A (probably) working version would be:

function li_mark(cell) {
    var elts=document.getElementById("up").getElementsByTagName("li"); //store the elements we need in a local variable so we don't have to rewrite all that line (and the browser won't have to do the fetching job again), we don't care if they're L-type or I-type
    for(var i in elts)
        if(elts[i].className && elts[i].className.length>=8) //check if there is a className and it is at least as long as "uplink-X", otherwise we don't process the element and skip to the next
            elts[i].className="uplink-"+elts[i].className.charAt(7); //we retrieve the letter for the type of class (L or I), and we append it to "upload-" to redefine the class of the current element
    if(cell && cell.className && cell.className.length>=8) { //if we passed an argument, then it must be a cell to activate, with a basic check on its className
        cell.className+="-active"; //after the loop, all elements have their default class, so we just need to append "-active" to the one we want to be active
        mark(); //we call mark() with no argument to reset all the items managed by mark(), we call it in this if-block to avoid infinite recursion
    }
}
function mark(cell) {
    var elts=document.getElementById("nav").getElementsByTagName("li"); //store the elements we need in a local variable so we don't have to rewrite all that line (and the browser won't have to do the fetching job again), we don't care if they're L-type or I-type
    for(var i in elts)
        if(elts[i].className) //check if there is a className, otherwise we don't process the element and skip to the next
            elts[i].className="link"; //we set back to the default class
    if(cell && cell.className) { //if we passed an argument, then it must be a cell to activate
        cell.className+="active"; //after the loop, all elements have their default class, so we just need to append "active" to the one we want to be active
        li_mark(); //we call li_mark() with no argument to reset all the items managed by li_mark(), we call it in this if-block to avoid infinite recursion
    }
}