Copy the link of the picture and then click the little icon that looks like two mountains on the top line where you post (with the bold, italic, underline icons) and then a pop-up should come on where you just paste the link and then post away! Apologies if it wasn't well explained.
This is the message I keep getting The following errors occurred with your submission: Required attachment missing
I'm assuming that you're posting in the fanart section because of this error. When making a thread in certain sections of the website you need to attach a picture to be used as a thumbnail for your thread. On the screen that opens up when you make a new thread you'll see a button that says "Manage Attachments" near the middle of the page; click on this and it'll open a window for you to upload pictures from your PC into. And in the future, you'll want to post threads like this where Mandiprime told you to:
Upload image to imgur: the simple image sharer Use <img>IMGUR LINK FOR IMAGE</img> where you want the image to go Relplace < and > with [ and ] Praise me as some form of deity Repeat as necessary
If it says "Required Attachment Missing" it means you are posting in a forum that requires you to upload at least 1 photo directly to the post. when posting, scroll down a bit until you see the attachment dialogue box and upload at least 1 image there.