Several times my upload of images failed, no matter what file format.
Prezis error message is a boilerplate: “check internet-connection”, “upload failure”, …
My messages are in german of course, because the language setting in Prezi is “german”.
The solution was relatively simple in this case, the filename contained a german umlaut (like “ä”), so I had to transliterate it. Curiously enough, the error message itself contains umlauts, so it took more time than necessary to discover the reason why the upload failed.
Please build more expressive error messages into Prezi!