8.1 you could hit "open in browser" on WPCentral App and upload directly. Pre 8.1, best way is to upload to Grin or similar App and embed the link. Grin is easiest as it gives you the link at upload, IMO.
I do not see and option directly in the WP Central App. It does not have an option directly to post an image from your phone.
1) Open thread in IE and add the image that way
or
2) Post your image to whatever site you are going to use and copy the link and paste it into the thread.