Hi,
I had the same problem. Not sure if you solved this by now, but here's what you need to do:
- open your form and go to design view
- double-click on your treeview control
- under the General tab, you'll see an option called "ImageList". There's a drop-down box where you should see your imagelist. Select it and hit OK.
This is how the treeview control now recognizes the imagelist you created.
Cheers!