The Workflow 4.0 Custom Activity Icon (and Toolbox Icon) FAQ
[Update: This FAQ is outdated and has be rewritten and reposted here:
https://blogs.msdn.com/b/tilovell/archive/2011/08/12/wf4-toolbox-updating-the-wf4-toolbox-icon-faq-for-rehosting-and-custom-activities.aspx
I'm leaving the original comments here.]