Thank you for writing.
Putting PictureBoxes in RadGridView will not make the grid load the images asynchronously. Additionally it is not recommended to nest controls within RadControls. RadControls are composed of elements and it is preferable that you nest only elements within them.
What you can do, is to show the picture in a screen tip or create a custom cell element that would load the images asynchronously. I have attached a sample project which demonstrates how a custom cell element could load the images asynchronously and cache them.
I hope this will be useful for you. Should you have further questions, I would be glad to help.
the Telerik team