If you post your photo as the avatar, it will appear as a thumbnail size photo, but if someone clicks on it to enlarge, it doesn't. For photos that you want us to see large, they must be attachments. While composing a message, look for the little paper clip among the icons above the text box. If you click on this, a window opens which allows you to browse and upload your photo(s) as attachments to the message.
Take note of the limits on photo dimensions. If too big, the upload will fail. So, you minimize the window and go to your photo program and resize the ones you want to upload. Now reopen this window and try again.
Photos that are attached to your message can be blown up to full screen by those reading your message. Let me add one here:
