The easiest way is to copy your picture ( right clic + copy) then to clic inside the message editing and (right clic + paste) paste it. The picture will upload into the post. Welcome to Steemit
Another solution is to upload the image somewhere and paste the url of the picture inside the message.
https://pixabay.com/fr/vol-%C3%A0-voile-f-lente-2138377/