Type in < img src="" width="400"> without the space after the < and put in the location (URL) of the picture you want to post, such as https://pbs.twimg.com/media/GTbqMS7XMAAsc65?format=jpg&name=large
I just grabbed the pic off X-Twit. Just right click on the pic you want to use and click "Copy image address". It automatically copies the URL and then you can paste it in between the quotation marks.
The width number can be anything, but I usually use 400 or 500.
It will look like this (but without that space after the < ):
< img src="https://pbs.twimg.com/media/GTbqMS7XMAAsc65?format=jpg&name=large" width="400">
And show up like this:
If you have any questions or need more clarification, just ask on thread or FReep mail me. :)