We recently included more tags support for HtmlString attribute, so, could you try our latest version/fix:Aspose.Cells for .NET v7.3.0.4 if it works fine with tag?
If it is not supported yet, we will log your feature request and support it soon.
Since as we told you that we might not support tag for Cell.HtmlString attribute or support it soon, I think you may use other Aspose.Cells APIs regarding insert pictures if it fits your needs, see the topic on how to add pictures to cells using Aspose.Cells APIs.