Hi,

There are lot of samples  for the same inside the odoo itself. To add the image in header you can see how the company logo is shown in the report header.

For adding image to report, try this:

<span t-field="line.image" t-options='{"widget": "image"}'/>

or

<img t-if="company.logo" t-att-src="image_data_uri(company.logo)" class="float-left" alt="Logo"/>

Also you can check this video, in it, it is showing how to add image in pdf report: How To Create PDF Report in Odoo

Thanks