The documentation for XLS and XLSX both have examples of Import/Exporting Images to an excel file. Please first check that these examples work for you:
http://reference.wolfram.com/mathematica/ref/format/XLSX.html
If you want to import images, the syntax is:
Import["ExampleData/cities.xlsx", "Images"]
The syntax for exporting an image to the file is:
Export["images.xlsx", {{"Multiple Images"}}, "Images" -> { image1Here, image2here}]
