1) If you've selected a theme that has a generic header image and want to replace it, first click on the header image and an orange EDIT IMAGE button will appear. Click that and take note of the Header size listed on the upper left-hand side of the page. (It will be listed in "px," or pixels, but if you don't have a photo editing program which can measure in pixels, use a ruler and measure the dimensions directly on the screen.)
3) Put the file in an easy-to-locate place on your computer.
4) Back on the EDIT IMAGE page, at the top right corner of the header image, you'll see a red circle with an X through it. Click that.
5) You'll end up with a blank space. Click it, browse to select the image from your computer, and wait for it to upload.
6) If you've uploaded a file larger than the default header, you can adjust its size by pulling/pushing the dots at the corners, place your cursor on the image itself to move it around. When it's in position, crop it by clicking the orange OK button.
*NOTE: sometimes the Weebly interface becomes non-reactive. if it does that, CLOSE out and re-select EDIT on your weebly home page.