Problems with image

It looks like you've added this code to create a drop shadow on all your image blocks. Because the box-shadow is targeting the image block (which is rectangular) you can see the outline of the rectangle. 

.sqs-block-image {
    box-shadow: 0px 4px 8px rgba(0,0,0,.13);
    transition: .3s
}

If you don't need that effect then you can just remove the code.

If you do want a drop-shadow to apply to the image then you have to take a slightly different approach because the way Squarespace creates the image shape prevents box-shadow from being applied, even if you targeted the image directly.

This code will create a drop-shadow to match the shape of the image:

.sqs-block-image {
    filter: drop-shadow(0px 4px 8px rgba(0,0,0,.13));
    transition: filter 0.3s;
}