Im trying out Blocs to see how I like it. Some aspects seem to be pretty cool but the images you have are generic and I can’t drag out images from Photos and when I put in an image from the desktop It tossed up a dialog stating it was not optimized for the web and it needed to be 3mb or smaller. The other issue is the image placed into the placeholder was placed full size instead of within the placeholder. Like the old iWeb the images in those placeholders were IN the placeholder and constrained within those boundaries and I could then move the image around to suit my taste and needs. It was like a built in crop but i could also select the image and move it around so the best part fit. The other issue is I found no place where I could add images for a project within the area like you have your generic images. Where is there a folder to place images.
You should always optimize your images. There are lots of both online services and Mac apps that will do this.
Mac App Optimizer (free)
Mac Apps (paid)
Free online Optimizer
These are just samples there are lots more.
As for Cropping, you can use something as simple as the preview(built in Mac) or a complex graphics app like Photoshop, Affinity Photo, or Pixelmator.
Picked up Squash from Realmac. Works great.