As this page shows, not every header needs to have an image.
If you don't have lots of good imagery and your microsite is an informative text-based website, consider not adding a header image.
What else
can you do?
There's lots of things you can do, including:
If you have lots of text you could break up the wall of text by using a dropdown.
In fact, you can add as many dropdowns as you'd like!
There's a range of other components to choose from, such as image galleries, image-led links, text-led links, large image-based call-to-actions and more!