When you upload images in ahead, there are a few things you should keep in mind to obtain the best results.
In order to also provide a good download experience for users using a mobile device, uploaded images will be reduced in size. This means that some loss in quality can be expected in favor of improved download times. For images of people and things this shouldn't pose much of a problem. For screenshots of e.g. a powerpoint slide, consider uploading the image as a png.
When you upload an image that has a width of less than 750 pixel, you will get some light grey borders that will center your image in the text. Above that size, the image will fill the available space while maintaining the aspect ratio. This means that your images shouldn't be too long, as they will then cover a large area of your first page impression (unless that is the effect you're aiming for, of course).
One useful tool to get a feeling for the optimal image sizes is placeholder.com. You can use it in ahead by putting an appropriate URL into the URL field of an image building block.
So, to preview an image of 750 x 290 (width x height) you use the URL https://via.placeholder.com/750x290.png.
The first image of the News post becomes the lead image which will be shown in the News feed on the homepage. There, the image will be displayed as a square in order to maintain a clean News feed.
This means that any image which does not have an aspect ratio of 1:1 will be cropped in order to fit the target aspect ratio of a square. This isn't usually a problem. Technically, the css directive applied to how the cropping is done is called object-type: cover. It will fit an image of any aspect ratio to the square of the News feed by focusing on the center of the image and cropping the borders until the aspect ratio is reached. You will notice, for instance, that with the placeholder images (see above), you will always see the text of the image because the text is always centered.
Unfortunately, this means that if your image has an aspect ration that deviates a lot from that of a square and the important aspects of the image are not located in the middle, the crop will not turn out optimal. Hence, you can optimize your lead image by paying attention to what can be seen in the middle of the image.
We hope this article helps you understand how best to use images in ahead. While the rules may seem limiting at first, we feel that keeping it simple helps you in the long run to provide content that can be consumed easily and looks good on all internet-capable devices.