I have Muse Blank Slideshow Widget at size 1049px x 133px with and used 3 random placeholder images while I styled the Widget.
I then proceeded to set up my real images for placement into the slideshow.
The Muse document is set to High DPI so I made an image at size 2098px x 266px. Exactly double the size. Then saved as JPEG.
When I imported the image into the widget by replacing one of the placeholder images, it was off-centre and required moving and scaling to size in the widget.
When I previewed in browser the new image was pixellated as if it was not a HiDPI image.
I decided to simply place the image in the document and Preview in Browser to see what was going on: