Header image size problem

Hello,

I'm a newbie at this and have a basic question. I'd like to insert my own image where the logo.png image is (the flower to the left of the blog title). I know how and where to upload the image, but can't get how to size it. I'd like it to be the same size as the flower was. I've resized the original image in Photoshop to about 3/4" tall, but when I upload it, it's far bigger than that. Is there a setting in the "Header Area" of the Options that I just don't understand?

Thanks.