You may want to set the image replacement strategy when replacing a placeholder, or an existing image, with the selected image.
This will provide a correct fitting of an image, when its dimensions are different that the placeholder ones.
On the General subtab of the Extra tab on the Preferences dialog box, tick Check if the caption frame is selected when dropping an image.
Recommended: ticked.
The behavior is described in Controlling image replacing behavior on page.
Notes
•This functionality uses the variable F_ImgReplaceOptionCmb.