tracking pixel

[kwadrat]

I've got a question about tracking pixel

Code: Select all

<img src=3D"http://mailing.example.com.pl/g.html?uid=3D1.b.5.0.hdhfd2b0c=
1" alt=3D"" border=3D"0" height=3D"1" width=3D"1">

what for is the URL? am I required to put any image there to be shown in html verison of the e-mail?