I can´t upload files to the forum.

I assume you mean upload using the "Attach a file", that feature does not seem to work i'm afraid (It keeps saying uploading...). I would instead recommend you to use an external image hosting service such Imgbox.com, imgur.com or similar. Copy the direct image url and insert it into the comment box using the "insert image" icon and press submit.

If the image appear then you have added it correctly, if it does not, then you might not have copied the direct .jpg/png/gif url.

Try again until you see the image appear in the comment box. 

I hope it helps.