Welcome to CYS!
Here are a few tips to add pictures and format them, but it only covers the basics so let us know what you're attempting to do if you need more help.
The simplest way is to use html. Copy and paste <img src="insertpicturehere.jpg"> wherever you want your picture to be, and replace 'insertpicturehere' with the image address. This works for most images online.
If your picture is downloaded, you can use an app like imgur.com to upload it, then right click the image and get the image address from there.
There is, of course, another way: go to the 'My Stuff' section on the left hand side of your screen, then click 'pictures'. This allows you to upload images and later add them to your storygame. However, there is a size restriction for the files you can upload, and as it's a much more inconvenient process, I wouldn't use this method unless you're uploading a profile pic/ picture for an item in your storygame.
I'm not much of an expert in scripting-like stuff, so if you're looking to do something complex like the examples Mizal listed, this probably wouldn't be useful. But for basic things like centering your image, you can use: <center> add your image here </center>. You can resize your image using style="height:800px; width:500px", while of course changing the numbers until your picture looks like you envisioned.
Together with the previous part, it might look something like this: <img src="insertpicturehere.jpg" style="height:800px; width:500px" />
For more complex formatting, you can browse through this list of articles and see if anything helps.
I noticed you've already figured out how to change colors and fonts in your test storygame, so I won't delve too much into this, but here's a helpful article for basic html and text formatting. It also covers images at the end if you're still confused about that.
Hopefully this helps, and best of luck with creating your story! Definitely check out the storygames Mizal mentioned to get a better idea of what the site is capable of.