[SalesForce] How to make an image publicly accessible

I wanted to store contacts profile pictures in the public folder and use the public check box to make them viewable on a wordpress website but the images always ask for a login to view.
Here is a screen shot of the uploaded file from the soap api.
/servlet.ImageServer?id=01511000000cmPQAAY&oid=00D1

the api code is as such

FileInfo fileInfo = new FileInfo(memberImageFiles.FullImageFilePath);
byte[] data = File.ReadAllBytes(memberImageFiles.FullImageFilePath);
Document document = new Document{
    Name = fileInfo.Name,
    IsPublic = true,
    FolderId = folderId,
    Description = " upload of file " + memberImageFiles.ImageName,
    Body = data,
    Type = fileInfo.Extension,
    IsPublicSpecified = true
};
var curentUser = sfdcBinding.getUserInfo();
SaveResult[] result = sfdcBinding.create( new sObject[] { document });
Contact user = (Contact) getUserIdResult.records[0];
user.Main_Profile_Picture__c = "<img alt=\"" + fileInfo.Name + "\" src=\"https://c.cs18.content.force.com/servlet/servlet.ImageServer?id=" + result[0].id + "&oid=" + curentUser.organizationId + "\"></img>";

here is the image tag that it produces

https://c.cs18.content.force.com/servlet/servlet.ImageServer?id=01511000000cmPQAAY&oid=00D1100000C1nc6EAB

It it the last line. I ported this over from php and it worked in another sandbox but for some reason it doesn't work in this sandbox. Any help would be greatly appreciated.
Thanks
Pk