If you can get to the photos on your phone, from your computer, you can upload and attach photos directly to your message. There is no need for use of a photo hosting site.
First, if you have the options at the bottom of the message to "Post Quick Reply", or "Go Advanced", click on "Go Advanced".
Scroll down, and under "Additional Options", click on "Manage Attachments".
Navigate to the photo, select it, and upload it.
Then, back up in the message, click on the Paper Clip Icon, and use it to select the photo you just uploaded.