1. Always make a new Httpjob for each request.
2. You should set the job name by calling Job.Initialize.
This code works:
Sub btn_Click
Dim h1 As HttpJob 'no need to make it a global variable
h1.Initialize("dnimg", Me)
h1.Download("https://www.google.com/images/branding/googlelogo/2x/googlelogo_color_160x56dp.png")
End Sub