Build 4.9.0.30 of xlsgen has support for adding pictures in sheets using the WebP file format standard.

WebP has recently become mainstream enough so Microsoft added support for this file format in Excel in 2021 in subscription-based Excel 365, and also the licensed version Excel 2021.

WebP is both lossy and lossless, depending on the client application generating it, and can have additional features which actually make it better than anything else out there from a feature standpoint and from a file size standpoint. In short, it is better than JPEG, PNG, GIF combined.

worksheet.NewPicture(".\\pic.webp", 1, 1, 36, 16, 0, 0, 0, 0);