Below the message composition window, there are 3 light blue tabs:
(note that the "Poll creation" tab will only appear if starting a new topic, not if replying to an existing one)
Click on the one called "Upload attachment". You should see this:
Click on "Choose File" and find the photo or file on your computer, then click "Open" or "Choose". The file name will appear next to the "Choose file" button.
Then click on "Add the file". Depending on the file size, it may take a few moments to upload, then the file name will appear under "Posted attachments" above two buttons "Place inline" or "Delete file" (both of those are optional). When you submit your message (it's recommended to preview it first, and you should see your photo or attachment in preview mode), your photo or attachment will appear in your message.
File comments can optionally be added in the blank provided.
Attachments are displayed in reverse order. The one you upload first will be displayed last. If you wish attachments to be displayed in a certain order, be sure to upload the last one first, etc.
If a photo exceeds a certain size, a thumbnail will be automatically created. Click on the thumbnail (smaller) image to see the full-size version. If you think your photo appears smaller than you expect, this is why.
To post multiple photos, just repeat the process before submitting the message. You will get a list of "Posted attachments" showing all the ones you have uploaded. It's recommended that you preview your post before submitting it to make sure it looks the way you want it to look and to make any necessary adjustments.
If you choose "Place inline", you will see attachment tags (with brackets) appear in the message composition window at the point at which your cursor is at the time. If you want to place the attachment elsewhere within your message, you can cut & paste the attachment tag, but be sure to use the ENTIRE tag including all brackets. Thumbnails will not be created if attachments are placed inline, so please to do not upload very large photos and place them inline. Preview your post to make sure it's where you want it to be.
When sending a Private Message, there are no blue tabs. It goes directly to the attachment option and will look like this:
Otherwise, the process is the same.
Here are two examples:
