While you’ve started to build your own theme, you have to keep it in mind that every inch mostly of the theme, you have to create with your own element. If I think of a header image of my blog (my theme) i have to think here with elementary ideas. because in google page ranking it has a deeper significance, so why you try to adopt others things?
I just created a header image of my blog, so i make a blank image with black background .
Then created an image of mine which must be appropriate with the black background image (above).
lol, may be this picture looks funny, but you have to stick with the work after all,extracting the foreground object with GIMP image editor(or Photoshop), placing it to the black background image of earlier.Now some text effects according to your needs,lastly use some smudge tools to soften the edges. the final result is something like this:
Cool isnt it?