I tred to put a jpg picture in an Excel file and found the below information in the LibXL documentation:

int addPicture(const wchar_t* filename)
Adds a picture to the workbook. Returns a picture identifier. Supports BMP, DIB, PNG, JPG and WMF picture formats. Use picture identifier with Sheet::setPicture(). Returns -1 if error occurs. Get error info with Book::errorMessage().

void setPicture(int row, int col, int pictureId, double scale = 1.0, int offset_x = 0, int offset_y = 0)
Sets a picture with pictureId identifier at position row and col with scale factor and offsets in pixels. Use Book::addPicture() for getting picture identifier.

In szLibXL, the corresponding procedures look like this:

    Procedure ComAddPicture String llfilename
    Procedure ComSetPicture Integer llrow Integer llcol Integer llpictureId Real llscale Integer lloffset_x Integer lloffset_y
Shouldn´t ComAddPicture be a function returning an identifier for use in ComSetPicture?