Since its too long for me to comment :) I'll try n provide a solution for her here.
Header are actually determind by the theme you have chosen for your blog. Some have a simple theme using one that is readily available in blogger like Minima, others add a personal touch by changing the background (like in Isabelle's case) while others are created using CSS or HTML - which is a language on its on
What ever the case may be, a header can be too small or too big or not proportional (looks like it has been stretched) if you do not take into account certain aspects.
A normal Minima template will have a fix header of 660pixels in width. You can easily find this in the tab Layout under Page elements - click header (look beneath Placement, shrik to fit. If you have an image there, you have to remove it first before you can see the size). It will follow whatever height of the photo/picture that you have uploaded but the header will always remain 660pixels in width.
If the picture is too small you will have spaces at the sides. If it is too big then it might exceed your header border
My template is one which I have customs not only to my liking but to be SEO friendly :)
As such, my header is opened to 960 pixels wide (since I am fully utilizing all 3 columns). So whatever header I may wish to create for my blog, I must take into consideration that it has to be 960 pixels wide.
However my friend does not jive with technical stuff, so I'll just leave this advice for her :
Find a photo that is large in pixels. This will make it easy to adjust.
Crop or shrink the image so that it is within the width and height of the desired header image (a suitable one is 660pixels wide(width) to 200pixels in height)
You can easily find the "px" for pixels in any image editing software :)
Hope this helps
You might need a few uploads before knowing if the header looks good or not, but be sure to save them under a diff name each. For e.g if u hv uploaded header1.jpg, make sure u upload a new file with a new name. Changes will not occur if the same file name is uploaded