There’s an old-school ’90s HTML design trick that dates back to the dim and distant past before we had decent layout control in CSS. That’s “slicing” – chopping a large image up into multiple parts, then reassembling them in an HTML table.
If you slice your images in an email and the end user hasn’t loaded images what will they see? They’ll see a rectangular box – either empty or with the image alt text in it. And, if you set the background colour for the table cell, they’ll see that – but only when images are turned off.
If you’re sneaky, you can do clever things with that.
The same mail with images on:
Or like this, with images off:
And the same mail with images on:
(There’s more discussion in this reddit thread about it).