1.	Open the Word document, go to your cover page and then Page layout > Margins > Custom margins
2.	Choose 0" as the margin width for top, bottom, left and right
3.	Copy-paste your image into the page, resize it to fit the entire page. Click on this image, go to Page layout > Text Wrapping and choose 'Behind text'. Press enter so that your cursor is below the image on the cover page
4.	Go to Page Layout > Breaks > Section breaks > Next page. In the next page, for the rest of the document, reset the margin width to whatever you want
5.	Go back to the cover page, copy-paste the second image you want overlaid on the first image. Click on this image, go to Page layout > Text Wrapping and choose 'In front of text'
6.	Drag and move each image wherever you wish. You should be able to place one image over the other

This treats the cover page and the rest of the document as separate sections. To check how your document is sectioned, go to View > Outline and look for 'Section breaks'.