Don't know if you're on the app or the website. Via website, click the little paperclip thing just above where you post. Or click "manage attachments" down below. New window pops up. Pick the file(s) you want, hit upload, window closes, and you'll see the attachment show up below. It'll be there when you post.
Never used the iphone/ipad app, but via the android app, press the little "plus" button next to where you post, then press the three dots at the top right, then "attach a picture", select picture, and press. "upload as attachment".
While you're supposed to use distilled water, the dissolved substances in tap water should be so low that it shouldn't make a difference with the equipment we use. It'd take something magnitudes more precise to detect the difference. What matters is temperature.
That said, if your hydrometer is flat out sinking in tap water and it's not incredibly hot, then you, sir, have got yourself a faulty hydrometer.