The only place where one can talk about speed and diffrent image types is when loading the image, since you normally don't need to load millions of images one can (mostly) ignore the small performance differences the loading processes could have, i.e. I have no idea but I'd say it doesn't matter.
On the other hand there's the diffrent encoding. PNG is a lossless format i.e. it doesn't lose any quality of the image, whereas the JPG is a format that will lose quality (if you don't save it right).
The only advantage JPG has over PNG is that it's filesize is smaller for bigger images.
I'd suggest to use PNG, for the reasons that it's lossless and supports alpha channel (i.e. transparency).
