Thanks for the replies! I managed to fix this by applying the CSS code...
... to the image, and editing the max-width within the div section.
BUT now I have another image-related problem. When I try to resize the top image (logo), it automatically shrinks down to nothingness and disappears. For the second image down (ebook cover), I can shrink it down but not make it larger. Then I can shrink or grow the other images without any problems.
Here's a quick video showing you what I mean, with the red rings showing when I left click: https://youtu.be/wrn_Cf0_DW8 .
Any idea why this is happening and how to fix it?