then follow this navigation
Admin CP > Styles & Templates > Style Manager > click check box (theame) > Under Image Path (Title Image)
give a path and load the image on your server eg : images/misc/logo.jpg
other solution will be to use a plugin that automatically adjust image size which will make the page to load slowly and does not add any value
my method just needs 2-3 trials and take only 5 min very simple hope it helps