I didn't see that you were asking about Silverlight. Excuse me. In Silverlight 3 you have the WriteableBitmap class that gives you ability to edit pixels on the fly. Unfortunately, Microsoft has not yet resolved the security issue of editing pixels of an existing image, and thus you are only able to take a snapshot of a control or input the pixels yourself. Rick Barraza has two very interesting topics explaining the use of WriteableBitmap here
that may be useful in your case. If you share your reasons why you want to edit the pixels of an existing image, maybe we can propose to you a workaround based on our experience. Thank you for contacting us and we look forward to hearing again from you.
the Telerik team