There is a camera icon for taking a picture. Otherwise you need to use the attachment function for sending a picture.

I've tried everything AFAIK. The pictures appear inside the chat area but with a red cross on their upper right corner indicating that sending failed.

After you take or attach a picture to the Signal chat you haven't sent it yet; you can write a caption for the image or send it without. The red cross if for removing the image if you change your mind. At least, this is what popped up in my mind immediately after your description.