You can float pictures left and right easily enough.
Let’s say you want to have a left floated picture. Make sure it’s the size you want and then drag it into the editing window, in front of the first letter in the parapgraph where you want it to appear. Select the image, then go to the Format menu and choose HTML, Align Image Left. Preview and you’ll see your image.
The built in blog isn’t very clever when it comes to positioning images. Some of Elixir Graphics themes have a blog entry topper feature which lets you place the picture above the bog entry - looks really nice.
But if the ability to position photos more precisely is important to you then I’d investigate Stacks-based alternatives to the built-in blog. Things like Instacks’ Poster stack, Joe Workman’s Total CMS and Nick Cates’ GoCMS are all worth a look.