Getting Help
If you ask a question you will have short "How to Format" in the yellow box right beside the editor box:

The link formatting help (in the image above I've put the mouse over it) leads to a more detailed page with examples.
The editor field also has a toolbar:

Here you can also find a button to get markdown editing help:

Also note the help center button in the top menu of the page:

How to Format Code Using the Toolbar
- Copy some code to the editor field, separated by one empty line from the text above. Nevertheless, in the editor field it will be shown a spaghetti code without any formatting:

- Select the code with you mouse and click to the code button
{}
in the toolbar:

- Now your code will be indented by 4 spaces and the preview below the editor field will show nice code:

How to Add an Image Using the Toolbar
If you want to add a screenshot make it and save it as PNG or JPEG. Some PDF-viewers also allow you to export pages as PNG or JPEG. You can use any paint/image software (e.g. GIMP) to add additional information to the file. Currently TeX.SX does not provide making PDFs/images on the fly. So you have to compile the TeX code either using you own TeX installation or an online editor like overleaf. Use tex4ht
, latex2html
only if they are part of the problem, because mostly the will not generate the same result as your preferred TeX engine.
Place the editor where you want to add the image and click to the image button in the toolbar:


- Select the file you want to add. A preview of the file will be shown. You should verify, that it is the correct picture, before you click to "Add picture":

- Something like
[![enter image description here][1]][1]
will be added at the cursor position an something like [1]: https://i.stack.imgur.com/4AtI7.png
at the bottom of your question. And the preview below the editor field will show the image.

- Last but not least, you should replace "
enter image description here
" by an alternate text, e.g., "left aligned blackbox with centered caption text
".