Hello tia
This is because you are not uploading an image with proper dimensions of the box. The images are meant to fit the boxes because they are the same size for it to look like a good design. you may edit your images in a fix dimension using a white background which will have same width and height for every image.
or you may try this method below
Go to dashboard > appearance > customize > additional css
paste the code below in the additional tab at the bottom in case there is another additional css is added.

#innerpage-box #content-box .lz-gallery-images img {

height: auto;
}
doing this will show the original dimensions of the images which might not look good to you