Blog post aligned left

With block elements, it’s easier to target only image or video elements because they have their own blocks with specific classes. I’m assuming you are really using the block editor and not merely editing in the “classic” block.

You can exceed the post container margins by styling the element to position: relative; and setting the left rule to some negative distance. For example:

.wp-block-image {
  position: relative;
  left: -10rem;
}

Of course you will need media queries to properly manage appearance on small screens which have little or no margins to exceed.

Ideally, you should add an image size that is ideal for your post width so the browser is not forced to use a bigger image than necessary. Regardless, if you style an image’s container (not the img tag, but a figure or div that contains it) to a specific width, then specify the img tag within has a width of 100% (of its container), and assuming the container itself does not overflow, then you can avoid getting scroll bars on images.

Using the CSS editor is discouraged in part because any changes you make could be lost when your theme is updated. One way to protect changes is to create a child theme. Even easier for CSS is to place any custom CSS you want in the Additional CSS panel of the customizer. Rules placed there become inline styles on the page and thus can take precedence over equivalent rules on theme stylesheets. Your rules added there are more likely to be properly applied without having to resort to !important modifiers or fiddling with selector specificities.