Hi All,
Kindly help me on writing test class for below Standard Controller Extension. Currently I am getting 47 % code coverage.

Controller:
public class SpeakerControllerExtension {

    public blob picture { get; set; }
    public String errorMessage { get; set; }

    private final Speaker__c speaker;
    private ApexPages.StandardController stdController;

    public SpeakerControllerExtension(ApexPages.StandardController stdController) {
        this.speaker = (Speaker__c)stdController.getRecord();
        this.stdController = stdController;
    }

    public PageReference save() {
        errorMessage = '';
        try {
            upsert speaker;
            if (picture != null) {
                Attachment attachment = new Attachment();
                attachment.body = picture;
                attachment.name = 'speaker_' + speaker.id + '.jpg';
                attachment.parentid = speaker.id;
                attachment.ContentType = 'application/jpg';
                insert attachment;
                speaker.Picture_Path__c = '/servlet/servlet.FileDownload?file='
                                          + attachment.id;
                update speaker;
            }
            return new ApexPages.StandardController(speaker).view();
        } catch(System.Exception ex) {
            errorMessage = ex.getMessage();
            return null;
        }
    }
}

Test class: 

@isTest
public class Test_SpeaketControllerExtension {
     static  testMethod void testSave(){
         
        
        Speaker__c sp = new Speaker__c(); // Speaker__c is Standard Controller
        insert sp;
       
        ApexPages.StandardController controller = new ApexPages.StandardController(sp);
       
        SpeakerControllerExtension extension = new SpeakerControllerExtension(controller); // controller extension
        extension.save();
        
        PageReference pageRef = Page.SpeakerForm; // SpeakerForm is visualforce page
        
        Test.setCurrentPageReference(pageRef);
    }
}

Thanks in advance