Adding image at a particular cell in Excel Spreadsheet
How to: Insert text into a cell in a spreadsheet document (Open XML SDK)
You will need reference to OpenXML, Systems.IO.Packaging, System.xml and/or similar.
You may need to copy the related DLLs to your production at deployment (depends on your deploy method)
to insert image located at another site (if any and under your control), you may need to change IIS config for : Allow Access Control