Just like text width, image size varies according to the display device's technical characteristics, i.e. screen size and resolution.
If the image is wider than Fluid Topics text, then the image is resized by Fluid Topics. This avoids horizontal scroll bars.Note: Image resizing is proportional.
If the image is not wider than the text, it is displayed in full size in the reader.
When the image is resized, you can bring it back to its full size by clicking on the Display at actual size icon.
Here you can see that the image and the text are resized according to the device's screen size.