Hope, I understand correctly:
If you place an image which is in your layout larger than the original image, you have 2 alternatives:
• Accept it, that Muse upsamples the image to the needed size during output.
• Open your image in Photoshop or another image editor and transform it to the needed pixel size.
In both cases the image will loose quality, if the original is smaller then needed.
By the way: The answer you found doesn’t meet your question! If it is your question, which should be answered, my answer was the correct one!
Why? Because your question deals with „missing assets“, not with upsampling.