Build 4.5.0.90 of xlsgen adds a new property to pictures, the ability to set a RGB-based chroma key in order to make transparent all pixels in that picture with that RGB color. This works with all Excel files.

Here is how it works. Assuming you have a picture to be inserted, where you know in advance that the red color is used as chroma key :

(C++ code follows)

xlsgen::IXlsPicturePtr picture1 = worksheet->Pictures->Add( L"input\\a.gif" );
picture1->LeftColumn = 1;
picture1->TopCell = 1;
picture1->ChromaKeyColor = 0xFF0000; // RED chroma key



Setting the chroma key color lets one see through the picture