Hi! If you click on 'reply' under this very post you're reading this second, the post editor should open, where you can the click on the icon I showed you. Then it will open your photos. You need to select a photo and click 'ok' or 'done' depending on your phone. It will then create a string of text that looks like a link (because you've effectively uploaded your photo). This should stay in the post you're composing. If you then press the send button, your photo will appear automatically for everyone to see.
PS, Sharing directly from your internal phone storage like you described works for things like text messages (if you get credit) and WhatsApp etc.
Hope that helps?