It’s under settings for you image elements. You can just select the image and the click the gear icon at the top right to view those settings.
Well, if you’re trying to navigate them to Final Designs at the bottom of the page, those images will have to load at some point anyway.
It’s a trade-off you have to choose, but if you’re wanting to use anchor navigation, you have to make the page layout predictable.
A better, but more intense way, is to specify the image dimensions of every single image on the page, so that the browser knows the layout sizing before the images are loaded. However, you have a lot of images and in some cases ( images inside of rich text elements for example ) you may have complications controlling those sizes directly in a responsive way.