DotNetNuke provides a class which provides the funktionality to get the object representing a file stored.
This enables us to easily store, retreive and assign files to links, images or others.
var image = (FileInfo)DotNetNuke.Services.FileSystem.FileManager.Instance.GetFile(FileId);
if (image != null){
img_Image.ImageUrl = DotNetNuke.Services.FileSystem.FileManager.Instance.GetUrl(image);
}
Indeed changing the application pool in which the DotNetNuke is working, from 'ASP.NET V2 AppPool' to 'Classic .NET AppPool' solves the problem. Changing the application pool to classic mode, you may use Coolite ASP.NET/AJAX Components in DotNetNuke. However this problem doesn't appear in 'ASP.NET V4 AppPool'.