We have checked the reported issue, the image rendering issue occurs in WebKit rendering engine itself. WebKit rendering engine is a old legacy which has known rendering limitations.
So, we are suggest you to try the blink rendering engine. Our Blink rendering engine internally makes use of chromium executables in headless mode for converting HTML to PDF. The converter will preserve the output PDF document, like how the input HTML file or URL is displayed in chromium-based web browsers (chrome).
Please refer below link for more information about the Blink rendering engine.
Blink: https://help.syncfusion.com/file-formats/pdf/convert-html-to-pdf/net-core
NuGet link: https://www.nuget.org/packages/Syncfusion.HtmlToPdfConverter.Net.Windows/
We have attached the sample and output for your reference. Kindly try the sample on your end and let us know the result
Sample : https://www.syncfusion.com/downloads/support/directtrac/general/ze/Htmlnet6.0-1003751331
Output :https://www.syncfusion.com/downloads/support/directtrac/general/ze/Output287675809