Hi,

For that first, we need to set the fields for images, also we can set the colors, or style as per our needs,

In the py:

from odoo import api, fields, models


class ResCompany(models.Model):
    """Introduce watermark in pdf reports"""
    _inherit = 'res.company'

    watermark = fields.Boolean(string='Watermark', help='Enable it, while you '
                                                        'want to apply '
                                                        'watermark on your '
                                                        'pdf reports')
    content_text = fields.Char(string='Text', help="Have the text here")
    watermark_type = fields.Selection([
        ('image', 'Image'),
        ('text', 'Text'),
        ('logo', 'Logo'),
    ], default='text', help='Select the watermark type')
    color_picker = fields.Char(string='Color Picker', help='Select the Color')
    font_size = fields.Integer(string='Font size', default=30,
                               help="Mention the font size")
    background_image = fields.Image(string='Image', help='Select the image')
    rotating_angle = fields.Float(string='Angle of Rotation',
                                  help='Mention the angle of rotation')

Then set the View, we need to define the XML file 

<?xml version="1.0" encoding="utf-8"?>
<!--Apply the watermark to the pdf report with mentioned specifications-->
<odoo>
    <template id="pdf_watermark_report"
              inherit_id="web.external_layout_standard">
        <xpath expr="//div[hasclass('row')][last()]" position="after">
            <t t-if="company.watermark">
                <center>
                    <div class="row"
                         style="opacity:0.15; font-size:100px;text-align:center;top:500px;position:fixed;z-index:99;">
                        <div class="col-6" name="watermark">
                            <span
                                    t-attf-style="-webkit-transform:rotate(-{{company.rotating_angle}}deg);color:{{company.color_picker}};font-size:{{company.font_size}}px;text-align:center;position:absolute;text-align:center;opacity:0.9;align:center;width:880px;">
                                <t t-if="company.watermark_type == 'text'">
                                    <t t-esc="company.content_text"/>
                                </t>
                                <span style="width:880px;align:center;">
                                    <t t-if="company.watermark_type == 'image'">
                                        <img t-if="company.background_image"
                                             t-att-src="image_data_uri(company.background_image)"
                                             width="250" height="200"
                                        />
                                    </t>
                                    <t t-if="company.watermark_type == 'logo'">
                                        <img t-if="company.logo"
                                             t-att-src="image_data_uri(company.logo)"
                                             width="250" height="200"/>
                                    </t>
                                </span>
                            </span>
                        </div>
                    </div>
                </center>
            </t>
        </xpath>
    </template>
</odoo>

Hope it helps