prova così:

function apri(ind,nome,larg,alt) {
var features = 'width='+larg+',height='+alt+',toolbars=no,menubar=no,location=no,status=no,scrollbars=yes';
window.open(ind,nome,features);
return false;
}

...

onSubmit="return (checkData() || apri('', 'UsrUploadAvatar', 300, 300));"