iMessage for Mac not downloading and not sending pictures

Hi everyone,

I have a problem on my iMessage for Mac. Basically:

1) It doesn't download pictures sent to me as attachments: although i can see the preview of the picture in the chat window, i can not save it nor see a bigger preview with the spacebar. The picture just isn't downloaded in the iMessage attachment folder, so when i try to open it with the spacebar or to open it with Preview or to save it, it opens another picture. Like this: i try to open the last image sent to me, it opens ANOTHER image sent in the same chat window some time ago.

2) I can not send pictures at all: if i try to send a picture via iMessage for Mac, whoever the picture is sent to says that a window says "Download" but the image just won't download.

How can i fix this problem?

i have already tried to restart my MacBook and i have already logged out of iMessage and logged in on my Macbook and on my iPhone, nothing changed.

Thanks