• obfuscating urls from the javascript side will not stop any dedicated individual

• var obfuscated = [] is considered better than var obfuscated = new Array();

• It is considered better form to have a single var statement

• reverseReg is an unfortunate name, it is unclear to me what it is supposed to contain

• Don't assign onsubmit, use addEventListener instead

• A loop that will only run twice should get unrolled ( for (i = 0; i <= 1; i++) { ), I would put the code in the loop block inside a helper function

• You have some indentation trouble, consider using jsbeautifier

• obf_index++ increases obf_index after evaluating it, so you can do

    for (i = 0; i <= 6; i++) {
        obfuscated[obf_index++] = refArray[i];
        obfuscated[obf_index++] = reverseReg[i];
    }
  

• Since you start from 0, you are really simulating push, so you could also do

    for (i = 0; i <= 6; i++) {
        obfuscated.push( refArray[i] );
        obfuscated.push( reverseReg[i] );
    }
  

• lowerCamelCase is preferred in JavaScript so obf_index -> obfIndex -> obfuscatedIndex

• You use both single quotes and double quotes for strings, stick to single quotes

• Use JsHint.com, you are not declaring every variable with var

• obfuscating urls from the javascript side will not stop any dedicated individual

• displayContent is an unfortunate name for the function, it does not give away that all it does it setting a listener, consider setFindCarListener

• var obfuscated = [] is considered better than var obfuscated = new Array();

• It is considered better form to have a single var statement

• reverseReg is an unfortunate name, it is unclear to me what it is supposed to contain

• Don't assign onsubmit, use addEventListener instead

• A loop that will only run twice should get unrolled ( for (i = 0; i <= 1; i++) { ), I would put the code in the loop block inside a helper function

• You have some indentation trouble, consider using jsbeautifier

• obf_index++ increases obf_index after evaluating it, so you can do

    for (i = 0; i <= 6; i++) {
        obfuscated[obf_index++] = refArray[i];
        obfuscated[obf_index++] = reverseReg[i];
    }
  

• Since you start from 0, you are really simulating push, so you could also do

    for (i = 0; i <= 6; i++) {
        obfuscated.push( refArray[i] );
        obfuscated.push( reverseReg[i] );
    }
  

• lowerCamelCase is preferred in JavaScript so obf_index -> obfIndex -> obfuscatedIndex

• You use both single quotes and double quotes for strings, stick to single quotes

• Use JsHint.com, you are not declaring every variable with var

• refArray is like reverse polish notation, postfixing the type of the variable, regChars would be a better name, it tells me that this contains the characters of the reg(istration)

All in all ( this is untested ), I would go with something like this:

function addCameraEntries( array , cameraSize ) {
    array.push( cameraSize + "/f" );
    array.push( cameraSize + "/i" );
    array.push( cameraSize + "/6" );
    array.push( cameraSize + "/5" );
    array.push( cameraSize + "/4" );
    array.push( cameraSize + "/r" );
}

function setFindCarListener() {
    
    document.getElementById("findCar").addEvenListener( "submit", function onSubmit() {
        var registration = document.getElementById("regPlate").value,
            reference = document.getElementById("stockRef").value,
            referenceChars = reference.split(''),
            ninthReference = referenceChars[10];
            registrationChars = registration.split('').reverse(),
            obfuscated = [],
            camera = [];
        for (i = 0; i <= 6; i++) {
            obfuscated.push( referenceChars[i] );
            obfuscated.push( registrationChars[i] );
        }
        obfuscated.push(ninthReference);
        obfuscatedString = obfuscated.join("");
        addCameraEntries( camera , '350' );
        addCameraEntries( camera , '800' );
        for (i = 0; i <= 11; i++) {
            var img = new Image();
            img.src = 'http://imagecache.arnoldclark.com/imageserver/" + obfuscatedString + "/" + camera[i] + "'
            document.body.appendChild(img);
        }
        return false;
    };

}