Thread:Project:Support desk/Image files over 1200x1600 or 1600x1200 uloads but fails to create the Image:xxxx.jpg file/reply

Note, your wiki is seriously outdated. The current version is 1.16. 1.6 is ten versions ago.

The warning is controlled by $wgUploadSizeWarning, which you can set in LocalSettings.php

As for the error message, could you do the steps at Debugging to try to get the actual error that occured?

Thanks,