1) Select the image file from your PC, by clicking Scegli(Choose)...
2) The second choice must be "non ridimensionare l'immagine" (not resize the image)
3) The third choice must be "normale" (normal)
4) Then click "inserisci l'immagine!" (insert the picture!)
In the third line "Anteprima per forum (1)" (Preview for forum), click "Copia negli appunti" (Copy to clipboard).
At this point you simply paste the link in the forum and the image will be visible directly in your discussion in preview mode. By clicking on the preview image it will open in a new tab for a better view.