Hi Terminatrix,
If you are trying to insert a picture, you will need to add a placeholder in your post, by clicking on the Img (Image) beside the "Select Image file to attach" at the bottom of the post edit window.
Browse for the image file to attach, then click Img button to add the placeholder.
Note that as you hover the mouse over the buttons, information about the maximum file sizes is displayed in the pink box at the top of the edit window.
When you click submit, the file will be uploaded, and the placeholder will be updated.
You should be able to edit your post to add the placeholder.
