In HTML you can force a height and width of an image. So even though an image is not downloaded (yet), it will still occupy the forced height and width. In most cases this a good thing, because the positioning of the rest of your email remain the same regardless of whether images are there or not.
A big but...
There are cases when forcing a height and width is not recommended. Imagine having a large image on top of your email, below the image you have text giving further details. If the recipient does not display images in their email now, they might never see the text below the picture. Instead they will just see a large empty space and might never even get interested in your mail (this can have a large impact on the success of your email campaign
At the top of your email you probably want to remove the forced height and width. Essentially, you always want some text "above the fold" in your message that catches the reader's interest, even if images are disabled.
This is also why ALT attributes
on your images are very important as they provide you with an opportunity to describe what is in the image.
Automated in SendCastle
When you select images in SendCastle's editor, we will always set a height and width automatically. Then when you verify/schedule your mail, we will always give warnings if large images have a fixed height. Likewise, we will also give an overview of which images does or does not have a forced height and width.