When it comes to the layout of your article, you'll probably only need the following content blocks listed below. Below are some links to help you if you're still trying to get to grips with using them.
The text block is the cornerstone of your article's layout.
As a general requirement, the images you use in your article should be minimally 1000 pixels wide. There is no maximum size, but it isn't really necessary for the image's width to be wider than 1500 pixels. This is to ensure that the images don't appear fuzzy or pixelated on the average screen.
This is an alternate way to display images within your post, and it is especially ideal for images that form a collection or a series. Again, the rule of thumb of image size (stated above) applies.
Personally, when it comes to the Gallery Design, I prefer using either the slider or grid configurations.
Use it to create white space in your article.
I generally include it after every text block (do note, within a text block, there can be multiple paragraphs) or content block (e.g. image, gallery, audio, video etc.). I leave it at its default height when I use it for this purpose.
http://help.squarespace.com/guides/using-the-audio-block - used only if you have the actual mp3 file, or if its uploaded onto a cloud storage system
http://help.squarespace.com/guides/using-the-video-block - embeds either Youtube or Vimeo videos