[xelawho]

depends what you call difficult. you have to loop through them all, but that's no big deal:

function plus() {
var pics=document.getElementsByTagName('img');
for(var j = 0; j<pics.length; j++){
thepic=pics[j];
thepic.style.width= parseInt(thepic.style.width)+200+"px";
    }
}

function minus() {
var pics=document.getElementsByTagName('img');
for(var k = 0; k<pics.length; k++){
thepic=pics[k];
thepic.style.width= parseInt(thepic.style.width)-200+"px";
    }
}