I am currently testing your app for a DTC store launching soon. I wanted to know if there’s a way to add a field for users to include pictures or attachments in their post? 

Correct, while creating a new post, if you click into the Editor, you will see a little "+" icon.

Click the little "+" icon.

Select image, or text....If you'd like to add a video, you can simply, copy-paste a video URL from YouTube.

For example, here's what happens when I paste a YouTube URL into the editor:

Pretty cool, right?! :)

Are different post types available? 

Sure, you can add different types of content (videos, images, gifs...)

What options do users have to further personalize the tool via dev/css?

In short, here are the elements which can be customized even without Custom CSS Design Code

You can edit these elements via the Social King Store Admin App (in the Settings Tab)

In addition, all the CSS Code can also be customized via the same page (if you scroll down to the bottom)