You can't use the Bitmap from System.Drawing in VBA. If you right-click on the work "Bitmap" in
sampleImage = New Bitmap(Image.FromFile(sampleFileName))
and choose "Definition" you'll get "Indentifier under cursor is not recognized". Alternatively, click on the Object Browser (or hit F2) and search for Bitmap in "All Libraries" - it will find it, but there is nothing there to use or call - no properties, no methods.
You could try the techniques listed here Extracting Colors from Image loaded in userform | PC Review
] - or, as you have tagged your question "VB" use VB.NET and the System.Drawing DLL