Header Image too large - www.craigynoscastleweddings.com

In order to keep our webpage from taking a long time to open, we need to reduce the size of our Header (top) image. We currently place an image at 278kb into the relevant field and add some text. When we upload it with any other changes we note that the file then becomes /style/top.png but is an image of the original file and the text we added but is a huge 758kb. We cannot reduce the original image any more – as we cannot get it down below 200kb without it losing its shape – or are you saying that with an image manipulation program we can add a really small image and then stretch it to place – but how large will it increase to once uploaded. Our workaround was to amend the top.png file manually and then FTP it into place – and it now stands at 92kb – although when we amend the site it will re-amend no doubt.