Hi all,
Here's a take on a LVOOP image processing framework. It will try to detect round features with a certain diameter using cross correlation of a reference circle and the acquired image.
This example sources also exist as a continuously evolving example in the LabQT LGPL Open Source Library.

Here's the block diagram:

Have fun and report any bugs here! 
Br,
/Roger
Edit 1: Replaced the wrong example images