In some templates there is no option to add image before/above the header or header image because in the layout we do not have 'Add a Gadget' field before/above the header, so we can manually create this option as under;
Open Template----->Edit HTML------>Proceed

Now find the following code (you may use Ctrl+F to find this stuff)
<b:section class='header' id='header' maxwidgets='1' showaddelement='no'>
<b:widget id='Header1' locked='true' title='YOUR BLOG TITLE(Header)' type='Header'/>
Change maxwidgets to 2 and shoaddelement to yes
Save Template
Before:
After:
JOJO