Can I add an icon to a page header?

LopaAtl Unfortunately, you cannot do it using SharePoint default page header.

Here are 2 workarounds you can try:

1. Use emoji instead of image icon:

1. Edit the site page
2. Click near page title text box area
3. Press Windows + . (period) to open emoji pane
4. Add emoji as per your requirements 

Output: 

2. Hide default page header and add page title using SharePoint web parts

1. Create a blank site page without page header and title by following: SharePoint Online: Create a blank page without header & title 
2. Add a section with one column layout at the top of site page
3. Add Text web part inside the page section
4. Add image and page title text inside text web part - format the image (move/drag, resize, etc.) and change font size of page title text as per your requirements
5. You can add other web parts on page as per your requirements and publish/republish the page.

Output: 

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