I've been using Squarespace for months and have been quite happy with how it takes care of so many Web page and site details for me, while also affording me the chance to customize. (I have beat the heck out of adding CSS tweaks.)
Images were still driving me batty, though. Inserting images into a post and resizing them seemed inconsistent and problematic, but I worked through. I finally went to the community forum and knowledgebase sites Squarespace operates and dug around until I found the answer to of the biggest problems.
You can drag and drop from the New Content Block list into a post.
In the corner of a post, there's a + icon. Click it, and you get a list of the various kinds of content blocks you can add to a page. If you just click a block, it's inserted at the end of the post. This is a pain because you then have to drag an image (or other item) up through the post, and in Firefox on Mac, at least, it doesn't scroll for posts. (It does when you're editing a page's layout.)
You don't have to do that. Just click +, and then drag the block into the page where you want it to go . This doesn't let you set up a left or right float, but it will let you put a block in as an "insert" or "row."
Resizing an image requires using the left/right handles, not the bottom handle.
When you have an image in a post, the left/right handles resize it proportionately. The bottom handle crops it, after which point, the left/right handles crop left or right! This drove me nuts. To reset proportionate resizing, double-click the bottom handle.