In order to put attach it, it ought to be a "Valid file extensions: zip gif jpg png txt jpeg html htm sav pcx net ace", JPEG would be preferable for a screenshot. You should remove the spaces and add the extension .jpg to the file. The file should be less than 500k (500000 bytes to be specific).
If you're on OS-less-than-X, use Graphics Converter to convert the file to JPEG (or at least reduce the quality in order to make it smaller).
If you're on OSX, just open it, it should by all means open in that Viewer app (can't remember what it's called) from which you can Export the picture to a JPEG.