[SalesForce] Controller method doesn’t rerender pageBlock

My aim is showing the pageBlock named iconGallery, when the user click on the commandLink "Show Gallery". If I put the "loadImage" method's code inside Constructor, everything goes perfect. So I wonder if the problem is between the Controller and VF page.

VF page:

<apex:page id="NewAchievement" controller="NewAchievement_Controller">

      <!-- Import Component -->
            <c:loadingSpinner />

      <!-- Page Header -->
      <apex:sectionHeader title="Achievement Edit" subtitle="New Achievement"/>

      <!-- Begin Form -->
    <apex:form >

    <apex:actionFunction action="{!loadImages}" name="loadImages" reRender="iconGallery" oncomplete="hideLoadingDialog();" />

    <apex:pageBlock title="Achievement Edit" mode="edit">

        <!-- Button Section -->
        <apex:pageBlockButtons location="top">
          <apex:commandButton value="Save" action="{!save}" />
          <apex:commandButton value="Save & New" action="{!saveNew}" />
          <apex:commandButton value="Cancel" action="{!cancel}" />
        </apex:pageBlockButtons>

        <!-- Fields -->
        <apex:pageBlockSection columns="1" showHeader="true" title="Information">
          <apex:inputField value="{!achievement.Name}" required="true" />
          <apex:inputField value="{!achievement.Category__c}" required="false" />
          <apex:inputField value="{!achievement.Points__c}"  required="true" />
          <apex:inputField value="{!achievement.Validation_Criteria__c}" required="true" style="width: 300px"/>
        </apex:pageBlockSection>

        <!-- Icon Selection -->
        <apex:pageBlockSection title="Select Icon">
          <apex:commandLink value="Show gallery"
                            action="{!loadImages}"
                            reRender="iconGallery"
                            onclick="showLoadingDialog();"
                            oncomplete="hideLoadingDialog();" />

          <apex:image value="https://eu6.salesforce.com/servlet/servlet.FileDownload?file=01558000000UI88" />
        </apex:pageBlockSection>
    </apex:pageBlock>

    <!-- Icon Gallery -->

    <apex:pageBlock title="Icon Gallery" id="iconGallery" rendered="">
                <apex:repeat value="{!allLinks}" var="item">
                    <apex:outputLink value="http://www.google.es">
                        <apex:image onclick="String" value="{!item}" style="padding-right: 10px; padding-bottom: 10px"/>
                    </apex:outputLink>
                </apex:repeat>
    </apex:pageBlock>
  </apex:form>
</apex:page>

Controller:

public class NewAchievement_Controller {

public Achievement__c achievement {get; set;}
public List<Document> allImages {get; set;}
public List<String> allLinks {get; set;}
public Boolean isClicked {get; set;}

public NewAchievement_Controller () {

    allImages = new List<Document>();
    allLinks = new List<String>();
    isClicked = false;
}

public PageReference saveNew() {

    try { 
     insert achievement; 
    } catch(System.DMLException e) {
        ApexPages.addMessages(e);
        return null;
    }   
    return (new ApexPages.StandardController(new Achievement__c())).edit();        
}

public PageReference cancel() {

    return (new ApexPages.StandardController(achievement).view());        
}

public PageReference save() {
    try {
        upsert(achievement);
    } catch(System.DMLException e) {
        ApexPages.addMessages(e);
        return null;
    }
    PageReference redirectSuccess = new ApexPages.StandardController(achievement).view();
    return (redirectSuccess);
}

public PageReference loadImages() {

     //LOAD EXISTING IMAGES (16x16)
    allImages = [SELECT Id FROM Document WHERE FolderId = '00l58000000hBuj'];

    String s = 'https://eu6.salesforce.com/servlet/servlet.FileDownload?file=';

    for(Integer i=0; i < (allImages.size()-106); i++) {

        Document d = allImages.get(i);
        allLinks.add(s + String.valueOf(d.Id));
    }

    isClicked = true;

    return null;
}
}

Thank you so much in advance!