Further inspection shows that this is purely a simple CSS issue - see https://www.screencast.com/t/G4DlvkWJ. Given what you see from the screenshot, what CSS do I need?
While I don’t completely understand how to develop a WP theme, the image is in the “masthead” section and I would think that this is something the theme controls. If you look at the SiteOrigin themes that do the exact same thing as Astrid, they allow any size image (I think you customize this “masthead” size) and then when you scroll it reduces the “masthead” and image about 50-60%.
Going to what I think is the root of this problem, when you tell the theme to select an image from your Media the WP Media function is governed by the controls of the theme. This seems to determine if the image needs to be cropped or if cropping can be skipped. In this case, the control says to crop because of the max size control in the theme. This is further demonstrated when you upload other images in the theme (i.e. header image) where there are no such controls and any size image can be used — thus Skip Cropping is allowed by the WP Media manager.
The CSS workaround clearly is to re-specify the correct original-sized image and change the width and height accordingly. Interestingly enough, when you do this, the “masthead” section automatically adjusts to accommodate the larger image as demonstrated by the fact that the 1px border under the section grows and that it scrolls properly when reducing the size of the section.