it’s like @Xyre@lemmus.org described. old content does not get federated unless there is new activity on it and someone on your instance is subscribed to the community it happens in.
this isn’t anyone’s fault, it’s just a matter of the instance not being around that long that some historic content will be missing.
photos will never be pngs unless someone intentionally converts them to that format, as pngs are much worse than jpgs for storing this type of image. pngs are much better for computer generated images, such as screenshots, drawings, etc. you can also losslessly compress pngs with tools like pngcrush without converting them to jpg.