Why are my images suddenly not appearing?

Greetings Heroes -- I've created a series of lessons for a client, reviewed them many times and today so many images are suddenly not rendering. It seems like the common issue is they're all jpg's .. I don't know for sure but I can see many png's are still showing. Some are from the content library and some are my uploads. 

Any idea why this might be happening? I really don't want to go make png's of everything and re-upload (there are a lot of modules in both French and English).

Thanks for your help!