You must add the "ftb.imagegallery.aspx" to the root of where you are using the Free Textbox:
            gallery.png

Open "ftb.imagegallery.aspx" file and modify properties to point to the folder where your images reside:

protected void Page_Load(Object Src, EventArgs E)
{
// *** remove this return statement to use the following code ***
//return;
string currentFolder = ImageGallery1.CurrentImagesFolder;

// modify the directories allowed

if (currentFolder == "~/ftb/images/")
{
// these are the default directories FTB:ImageGallery will find
string[] defaultDirectories = System.IO.Directory.GetDirectories(Server.MapPath(currentFolder),"*");

// user defined custom directories
//string[] customDirectories = new string[] {"folder1","folder2"};

// the gallery will use these images in this instance
//ImageGallery1.CurrentDirectories = customDirectories;
}

// modify the images allowed
if (currentFolder == "~/ftb/images/")
{
System.IO.DirectoryInfo directoryInfo = new System.IO.DirectoryInfo(Server.MapPath(currentFolder));

// these are the default images FTB:ImageGallery will find
System.IO.FileInfo[] defaultImages = directoryInfo.GetFiles("*");

// user defined custom images (here, we're just allowing the first two)
System.IO.FileInfo[] customImages = new System.IO.FileInfo[2] {defaultImages[0], defaultImages[1]};

// the gallery will use these images in this instance
ImageGallery1.CurrentImages = customImages;
   }
}
</script>
<html>
<head>
<title>Image Gallery</title>
</head>
<body>
<form id="Form1" runat="server" enctype="multipart/form-data"> 
   <FTB:ImageGallery id="ImageGallery1" 
   JavaScriptLocation="ExternalFile" 
   UtilityImagesLocation="ExternalFile" 
   SupportFolder="~/aspnet_client/FreeTextBox/"
   AllowImageDelete="true" AllowImageUpload="true" 
   AllowDirectoryCreate="false" 
   AllowDirectoryDelete="false" runat="Server" />
</form>
</body>
</html>

Remember to modify the permissions on the web server to allow Read/Write Permissions.