To help anyone that has problems uploading their images, and based on my experience in the Forum (I was helped, thank you!), here is a simple recipe:
To add an image to a post, the best way is follow three simple rules:
1-) Don't upload images bigger than 960px in its longest side.
2-) Don't upload images heavier than 500 kb
3-) After adding your images to the post ("Add File" button), click on the button "Insert".
Both buttons are located inside the "Attachments" pane in the "New Topic" window.
A new line of code will be created inside the typing area, and the picture will be embedded in the post.
Take a look at the picture below:




I hope it helps!