Problems with photos next to text

Hello

I am testing your WP books plugin except for one point it works fine. The problem is in the photo I want to add to the books I use the photo sizes of high x width 375 x 260.
When I have done that, the photo becomes slightly out of focus when I look at the dimensions of the photo, they have been changed to 300 x 218 mm. (Copy on the bleed screen) If I want to make the photo smaller, the photo remains unchanged next to the text.

Kind regards Hans