There is an "Insert an image" button next to the Youtube icon at the top.

I see that but it requires a url and I have an image on my Mac

It is a limitation of the forum, not to add attachments to replies, I will research a workaround.

I have the attachment button on a new post, but I do not have the same when replying to a post

The quick reply  area below the footer of the topic you are viewing does not have an attachment button.

To reply with an attachment, use the Reply button in the header or footer of the topic.

You can then either "add files" to locate the file you want to attach from your local disk, or from your file manager you can drag and drop the file into the box with the message "Drag and drop your files here, or use the button to add files."

Let me know if this is helpful, then we can sticky it to help other users.

Thanks