Right underneath the box where you type a reply is a gray box that says… select/drop/paste files here. Click it and navigate to your phones photos or files and select the one you want to post.
Then hit new reply.
Since obviously you are a new fag (we all were once)- check the preview of the uploaded photos or files before you hit “new reply”. You don’t want to upload any personal stuff.
Don’t dox yourself or your family.
Godspeed.