Muse converting jpegs to .pngs, ballooning size

Question

The photos in a Muse site I created recently were loading very slowly, so I uploaded the files into Dreamweaver to see what was happening. To my shock, here's what I found when I reviewed one of the photos:
The original (linked photo): a 640 X 480 jpeg (66KB)
The working Muse file (desktop): I resized the photo to 420 X 315 in the layout
The (desktop site) file created by Muse and downloaded to the server: resized to 429 X 324 (larger than I resized it), turned into a .png (!), file size increased (even though resolution is much less), up to 134 KB vs. 66 KB for the original
The working Muse file (phone): I resized the photo to 351 X 263 in the layout
The (phone site) file created by Muse and downloaded to the server: resized to 360 X 272 (larger than I resized it), turned into a .png (!), file size increased (even though resolution is less by almost half), up to 112 KB vs. 66 KB for the original
So Muse is converting some (not all) of my jpeg photos to .png, resizing upward slightly from the size I indicated in the working Muse file, and and almost doubling the file size.
OK, I know best practice is to resize and optimize outside of Muse, but what good will this be if Muse resizes the results and turns small jpeg files into large png files? And even without resizing and optimizing outside of Muse, shouldn't Muse NOT change jpegs to png's and nearly double their file size from the size of the linked originals (which themselves are nearly twice the resolution, and therefore should themselves be larger than the file size of the files Muse produces, not less)?
Could someone from the Muse team explain this?
Thanks.

Answer 1

Hi Zak,
Thanks for your quick response. The site in question is http://www.nycjazz.com. The photo I described can be found by clicking on the "vocalists" link and locating the listing for "Brazilian Jazz Quartet" that appears towards the bottom of the page, just above the contact form. It is a photo with no transparencey, but with a drop shadow applied
Your response raises a few more questions:
(1) If some browsers can acheive drop shadow effects using CSS and others can't, then shouldn't Muse render two sets of these photos—jpeg to be served for the modern CSS3-complient browsers, and .png to be served for the older non-CSS3 complient browsers, rather than one set of huge png's served to both, as seems to be the case?
(2) If Muse has to create huge .pngs for both under these circumstances (at least as it may stand for now), is there a workaround designers can employ to trick it to do otherwise? Like, for example, grouping the photo with something else and applying a shadow, or putting a transparent box behind the photo and applying a shadow?
(3) This behavior of Muse is inconsistent for some reason. For example, if you look at a photo just above, in the same "vocalists" section, for "vocal-piano trio," Muse created a .jpeg for the desktop version, and a .png for the phone version. Muse did do a decent job of opitmizing the jpeg in this case, without any effort from me, though; the linked original in this case is 640 X 480 and 46 KB, whereas the jpeg Muse created from this is 420 X 315 and 15 KB. What explains the inconsistency, though?
(4) One thing I don't comprehend is the notion that you can optimize photos in advance of placing them in Muse. How can this make a difference, if Muse, either way, is going to automatically optimize them itself using it's own built-in algorhyrhms when outputting them?
If you could answer each of the these four point in turn, it would be very helpful.
Thanks.