Dear Mr. Hernandez,
Thank you for your reply. The problem indeed is with jpeg files. I had no problem either uploading png images. But this is at the cost of the time taken to convert jpeg images into png format, the longer time needed to upload the latter and, at a user's end, to download them.
Therefore, the ideal would be to use the jpeg format first.
I would indeed be grateful if you could let me know whether this problem can be solved.
With my best regards in the meantime,