Changing CSS elements using Javascript

Each of these blocks

if (myCampaign == "street"){
   var elems = document.getElementsByClassName('row-background-79134');
    for(var i = 0; i < elems.length; i++) {
    elems[i].style.background = 'url(https://example.com/image1.jpg) no-repeat center center;'
}

Should be re-written with an extra brace, like this

if (myCampaign == "street"){
   var elems = document.getElementsByClassName('row-background-79134');
   for(var i = 0; i < elems.length; i++) {
       elems[i].style.background = 'url(https://example.com/image1.jpg) no-repeat center center;';
   } // added this
}

Also consider adding some logging. Use console.log to show you what's going on, and open your browser console to read the output. You probably want to know what the value of myCampaign is, for instance.

console.log(myCampaign);

will help, near the top. There are other ways to make the code a bit easier to read/maintain, but getting the braces right should be the first step.

Your code if statements are missing a closing bracket }. However, the major problem here is that your code is much larger than it should be, and is a redundant mess.

To fix this, it is a much better idea to store the campaign names and image values in a object, then have one if statement:

//This would be the value you get from the URL, I've replaced it with a prompt for demo purposes
var campaignName = prompt("campaign name? (leave blank for default)") || "street";

//Object containing a name/image pair for each of the campaigns
var campaigns = {
  street: "image1.jpg",
  plumber: "image2.jpg",
  cowboy: "image3.jpg",
  hairdresser: "image4.jpg"
};

//If a campaign with the name exists
if (campaigns[campaignName]) {
  
  //Select elements by class name
  var elems = document.getElementsByClassName('row-background-79134');
  
  //Loop through elements
  for (var i = 0; i < elems.length; i++) {
    
    //Since we have the other styles added with CSS, we only have to set the background-image
    elems[i].style.backgroundImage = 'url(https://example.com/' + campaigns[campaignName] + ')';
    
    //Logging for demo purposes
    console.log("Added image to", elems[i]);
  }
}

.row-background-79134 {
  background-repeat: no-repeat;
  background-position: center;
}

<div class="row-background-79134"></div>
<div class="row-background-79134"></div>
<div class="row-background-79134"></div>
<div class="row-background-79134"></div>

Much cleaner, simpler, and easier to debug.

If you want to add a new campaign/image, just add it to the campaigns object:

var campaigns = {
  street: "image1.jpg",
  plumber: "image2.jpg",
  cowboy: "image3.jpg",
  hairdresser: "image4.jpg",
  chef: "image5.jpg",
};

Try this, There are syntax issues in your code. You did not close your brackets properly.

if (myCampaign == "street"){
   var elems = document.getElementsByClassName('row-background-79134');
   for(var i = 0; i < elems.length; i++) {
   elems[i].style.background = 'url(https://example.com/image1.jpg) no-repeat center center;'
   }
}

if (myCampaign == "plumber"){
   var elems = document.getElementsByClassName('row-background-79134');
    for(var i = 0; i < elems.length; i++) {
     elems[i].style.background = 'url(https://example.com/image2.jpg) no-repeat center center;'
    }
}

if (myCampaign == "cowboy"){
  var elems = document.getElementsByClassName('row-background-79134');
  for(var i = 0; i < elems.length; i++) {
  elems[i].style.background = 'url(https://example.com/image3.jpg) no-repeat center center;'
  }
}

if (myCampaign == "hairdresser"){
   var elems = document.getElementsByClassName('row-background-79134');
   for(var i = 0; i < elems.length; i++) {
     elems[i].style.background = 'url(https://example.com/image4.jpg) no-repeat center center;'
   }
}

In each if bracket, youre missing a "}" which should close this block. In addition to that, next to the background assignment you are missing a ";",

wrong:

if (myCampaign == "cowboy"){
  var elems = document.getElementsByClassName('row-background-79134');
  for(var i = 0; i < elems.length; i++) {
     elems[i].style.background = 'url(https://example.com/image3.jpg no-repeat center center;'
  }

right:

if (myCampaign == "cowboy"){
  var elems = document.getElementsByClassName('row-background-79134');
  for(var i = 0; i < elems.length; i++) {
     elems[i].style.background = 'url(https://example.com/image3.jpg no-repeat center center;';
  }
}

If you want to make your current code shorter, use jQuery.

JavaScript:

var elems = document.getElementsByClassName('row-background-79134');
for(var i = 0; i < elems.length; i++) {
 elems[i].style.background = 'url(https://example.com/image4.jpg) no-repeat center center;';
}

jQuery

$(".row-background-79134").css("background","url('https://example.com/image4.jpg') no-repeat center center");