I just created a new story map using the same images at various sizes. Curiously, the first time I loaded it, the problem with the images being different sizes persisted, but the second time I loaded the page, the images were all the same, correct width.
The third time, the problem came back, and that's when I noticed a pattern. It appears that if I load the page when I'm holding my phone in portrait, the images load at the correct width and will go all the way to the edges of the screen in both orientations (although the bottom of the images are being cut off when I rotate to landscape for some reason), but if the images begin loading while I'm holding the phone in landscape, they load incorrectly. I tried loading the original story map in portrait and landscape with this in mind and got the same results as with the new story.