Replacing a prenamed image in a document

Hi

I have a task in where I recieve a document with pre-formatted named images and where I want to replace the imagepart in the document with a new picture.

I now I can change the image info with feeddata-functions for the imagepart, but my problem is to locate and identify the image.

In the image below I have cut out the run-part och the document where the picture is placed.
Here I can see that the name of the picture is Image_MainImage, and the relation ID of it is rId8.

Can anybody supply som code (VB or C#) on how to retrieve the relation ID?
With the ID I believe it should be easy to replace the imagepart.

Thanks in advance

Best Regards Peter Karlström Midrange AB, Sweden