Actually both .jpg and .png have their uses. PNG is perfect for small pictures, some other pictures that don't have a lot of color variation and for pictures that have transparency flags. Jpg is great for bigger and colorful pictures. Regarding the loss of quality, it depends on the software you use. I usually save my pictures with a quality of 8/10 in Photoshop and it's pretty good. The point is, I'd use Jpg over PNG for those specific pictures because they are much smaller in size in general (again, mainly for bigger and colorful pictures). It's a wiki after all and it's best to try and save on picture size or some pages may end up with several megabytes of data to be loaded which is huge! And with a quality ratio of 8/10, you'd barely (or just slightly) tell the difference between a Jpg and PNG, which is perfectly acceptable.
For icons (e.g. Plot legend), country flags, small character pictures, PNG is indeed much better.
Just a random example: a 640x480 picture could be saved in Jpg for about 50Kb. The same picture in PNG could be around 250 Kb or more! And the quality difference wouldn't be quite noticeable (given a good jpg quality ratio). It'd be even less noticeable (or not noticeable at all) if the source is not of perfect quality from the start (like it'd be if it was an HD episode for example).
To sum up, there's not really a standard at this point but the way I've been doing things, I prefer saving to jpg the following:
- Profile pictures, character pictures, people pictures, gallery pictures, ...
I use PNG for:
- Icons, flags, plot legend, small pictures, ... (they could also be used for the 60px version of characters which we could start converting to if we wanted to)