To attach a file (jpg, gif, png, xar or web file etc) Follow this 12 year old video. The procedure is unnecessarily complicated but can't be helped. No need to start a new thread either, just scroll down & select Manage Attachments (In fact you may also need to Hit an Advanced button these days to find that button, I'm not sure).

Looking at your screengrab the site looks quite large and may be too big to attach. If that the case just copy your index page, open a new web document and paste it then save as small-site.web or similar and you'll be able to attach that.