Crop your header in the right size
To be able to use the function "slide header" the size of your picture has to be 2880 pixels x 1100 pixels. The best thing to do is to upload your picture from the camera and then crop it with an editing program, such as Photoshop or Pixelmator. If you don´t have access to either of these two we recommend you to use the program Canva Photo Editor, which is a free online-program.
Canva Photo Editor
The first thing you have to do is to upload your photo and then use the function that will crop your picture. The width of your picture has to be 2880 pixels and the hight 1100 pixels. Otherwise the crop-tool won't work. When you have the right measurements, continue and press the button that says "apply", and save the picture.
When the picture is cropped in the right way, the proportions should look similar to this.
Add animated pictures
To be able to use your picture as a slide header, you have to change the settings in your design. Close your regular header and pick the new module with the slide header to your blog. After that you should be able to upload your new picture.