I don ´t see anything unexpected on your site. Allow me to explain some factors related to your observations though -
# First of all, there are two differences you should be aware of: 1. The startimage/filemod is not considered an imagefile like normal images in your gallery. When normal images are being displayed, Imagevue smartly resizes the images downwards to fit within the screen margins. For the startimage/filemod however, it simply loads the file specified, and will display it. Resizing the browser will NOT resize the image being used, and it will always display at its original size.
# The startimage/filemod is always centered. That is unless the stage area becomes too small, it will move downwards as to allow the description to be visible above the image.
Taking into consideration the two factors mentioned above, I don't see how your site differs in any way to other sites, for example our demo site with reacts the same way:
https://www.photo.gallery/soda/