Are there Linux/Ubuntu programs capable of inserting a partially transparent image into a PDF? I'm trying to "sign" a PDF document by inserting an image of my signature, but even though every OSX and Windows PDF editor seems to support this, I haven't found any Linux PDF editors that do. I've tried PDFChain, PDF Editor, Flpsed PDF Annotator, Openoffice, Scribus, Krita, and PDFSam, and none support this. Although not technically a Linux program, I tried the site pdfescape.com, but it corrupts the images it inserts, rendering it useless for this task.

Note, I'm talking about keeping the PDF in PDF format, so rasterizing it to a TIF/PNG/BMP, editing it in Gimp, and then dumping it back into a PDF isn't a solution.

EDIT: I might have been premature in my criticism of pdfescape.com and PDF Editor. I was viewing the resulting PDF in Evince, which was showing a mangled image, but when I opened the PDF in PDF Editor, the image rendered correctly. I've since sent the PDF to someone on Windows who confirmed the image showed correctly. It looks like the problem might be inaccurate rendering with Evince.