I found great resources for sanning in VB.net with a TWAIN compliant device.  I tested using an HP Scanjet 5590.

Check out these links:

This article has a great demo program to get you started.  The forum entries for the article contain valuable information.

http://www.codeproject.com/dotnet/twaindotnet.asp?df=100&forumid=2619&fr=51#xx1144564xx

This site helped as well:

http://www.accusoft.com/support/Documentation/ImageGear/13.2/Windows/Core/Activex/C7_api_reference_AX_v131360.html

Finally, Bob Powell has a wonderful site with lots of GDI+ related material:

http://www.bobpowell.net/