Any of the files in the pix folder that are part of the Certificate download work fine. When I create my own image, e.g., sampsig.png, and upload it, when I try to display the certificate with one of the graphics selected, I get this:
TCPDF ERROR: Can't open image file: /home/learning/moodledata/cache/tcpdf/mska_b36071b1c5fea591a0534c043083d3d4
The permissions on my graphics are identical to those included in the downloaded module.
I was successful in downloading a border image, editing it, saving it with a new name, uploading it, selecting it, and successfully displaying the certificate. It is a signature image that causes the error.