No transparency in custom stamps

Hi,

I've create a stamp in Adobe Reader. The only wau to create it is from a PDF file. I created 2 differents stamps from 2 differents files : 1 from jpg with no transparency backgroun, and another one from a PNG file with transparency background (no white in background). In two case, the stamp in not transparent ... I think it's because Adobe Reader need to import stamp from a PDF file, and not from a PNG file who support transparency.

Thanks for any suggestions!