1024 px on the longest side(s)
Images up loaded to your gallery must not exceed 1024px on the longest side, nor 350Kb
or you will get an error message.
If attaching a file direct from your PC / device, into a thread.
this must not exceed 1024px on the longest side or 500Kb
otherwise it will fail to up load.
If linking from flickr 1024 still applies but the Kb doesn't.