Columns are used to vertically arrange a number of widgets. In this case the image, the username, and the text. Along with those visible widgets there are also two SizedBoxes. These simply add a little space between the widgets. CrossAxisAlignment.start is used to left align the text.
The image is wrapped in a ClipRRect widget with a borderRadious to give it nice round corners. The Text widgets are using the parent theme styling so they will stay current with future theme changes.
This is only a basic implementation of rendering images but I've planned some future improvements like fading in and on device caching.
To add some space around all the PostItems, I'm updating PostsList to use a Container widget with some padding. I'm not putting the padding inside PostItem because it's the parents responsibility to position PostItems in relation to each other. EdgeInsets.fromLTRB will pad the left, top, right, and bottom edges respectively.
In the image URLs, I've told Unsplash to return images with a width of 400px. This is simply to avoid downloading multi-megabyte images in development. Once users are uploading images in Birb, there will be more control over image sizes.