Problem Uploading images

Hi,

I have the following code, and it works just fine on my local server, but when i try to use it in my hosting it bring an exception when its trying to save. Any ideas??

.
.
.

Try

originalimg.Save(Server.MapPath(upload_dir & UploadTest.FileName), originalimg.RawFormat)
thumb.Save(Server.MapPath(upload_dir & "t_" & UploadTest.FileName), originalimg.RawFormat)
Image1.ImageUrl = upload_dir & IdFalsa & UploadTest.FileName
Image2.ImageUrl = upload_dir & IdFalsa & "t_" & UploadTest.FileName
Image1.Visible = True
Image2.Visible = True

Catch

msg = "Lo sentimos, hubo un problema al intentar guardar la imagen."

End Try

.
.
. ...