To send a photo on a mobile phone, use the icons to the left of the text sending area. It is possible to send a picture that is already on your phone, or take a picture directly. In the latter case, be aware that the photo will remain in PetalMD, and will not be accessible in your phone.
To send an image or file from the web, use the paperclip icon.
Here, in video, how to send a photo or a file.