You're not really using the file system as intended with DNN. Images should be managed through DNN's DotNetNuke.Services.FileSystem namespace and you're likely writing a lot of code you don't need to in order to save images as well (try the dnnFilePickerUploader control, it handles the upload and passes you a FileId). Once you have the FileId persisted with your object, you could use something like this to load the image:
private string GetPath(int fileId)
{
StringBuilder sb = new StringBuilder("/Portals/");
IFileInfo fi = FileManager.Instance.GetFile(fileId);
sb.Append(fi.PortalId);
sb.Append("/");
sb.Append(fi.RelativePath);
return sb.ToString();
}