I think it is always somewhat worthwhile to work on having efficiently encoded and sized images. Not just for page loading, but for overall storage considerations. For something like hubpages, the viewable width is probably about 800 pixels or so (I don't know the exact width), so for a full width photo I will usually size the width to be in the 640-800 pixel range and then I save them as jpgs usually at the 85% quality rating. This will give you images in the 50-100kb range, which is, in my opinion, very manageable. It is worth playing around with a bit. You could take an image and try to make successive copies smaller and smaller and post them into a hub to see how they actually look online. If a 32kb image looks about the same as a 200kb image than you might as well go with the small one. (That would actually make a great hub if no one has done it.)
Personally, It doesn't concern me too much on Hubpages. Words, pop up right away and can keep someone busy even if an image takes a few seconds to load. Other sites which depend heavily on graphics and visuals it matters much more.
As for jpg vs. png I am a long way from an expert. My understanding of pngs is that they were intended to replace gifs. A primary use of gifs and pngs is for images which require transparency. But, absent that requirement, it is my experience that jpgs are generally much smaller than pngs or gifs, so I wouldn't convert for that reason. Also, there are still some platforms/browsers which do not handle pngs well.
It is also important to point out that lots of other things can slow down webpage loading times. Webpages are pretty complex these days and there can be all sorts of coding and scripts and activity running as they load.
There are websites and software programs which analyze webpage loading times and can offer suggestions for improvement, but I don't have any experience with them so can't recommend any, but they may be worth checking out.