It's actually quite simple. Let's suppose you want to put your article as a new thread under the 'Modelling Matter' section. If you go to that section, at the top left is the section title and under it in red 'New Topic' Click on that - if you aren't logged in you will be prompted to do so, then you'll be taken to a text entry box. Either type your text in that, or copy and paste it if you have a text already.
For photos you need a web storage site like Photobucket, which I use. After you upload your photos to Photobucket open the one you want to put in the text first and on the right side is a list of links to the photo - click on the bottom one and it says 'copied' - paste the link into your text. The the link should start with img in square brackets and end with /img in square brackets.
Once you have the text and photos entered in the box click on 'Preview' at the bottom to see how it will appear. If you need to correct anything the text entry box is just under the preview. Once you are happy click on 'Submit' and your article will appear in the Modelling Matters section.
Similarly, if you are replying to a post click 'Post Reply' at bottom left of the last post and proceed as above.
Other photo storage sites have other ways of generating the link but the principle is the same.
Looking forward to reading your article!