Hmm, yeah I am only 'short-sighted' so I could see quite a huge difference between  and  for the first set of images (forest).
For the 'MITTINEAGUE Park' I found it harder to read on  but didn't notice a lot of difference between  and . As for [strpng.png] that is pretty dire and looks like 8-bit.
Hmm, a web browser has no concept of dpi thus it will appear different to say Photoshop (where you can see print and actual size, etc.) As far as the browser is likely to be concerned anything over 72 dpi is meaningless anyway (or for that matter so is 72 but that's another story).
In reality the dpi 'Text Size' is a "Logical Inch" computed value. Don't confuse "logical inches" with "real inches" - very different concepts.
For example we have three; 100px by 100px images, one at 72 dpi, 96 dpi and 300 dpi. It will look the exact same size within a web browser but when printed it will differ.
Basically DPI does NOT apply to video screens; video systems know no concept of dpi at all or any concept of inches either. You should notice that the terms "dpi" or "ppi" simply do not appear in any user manual for any monitor or for any video board.
Hence why the file: strpng.png (400 x 300) is larger (File-size [KB]) than; strpng2.png (374 x 281) it is the dimensions you changed NOT the dpi (as such both are 2 dpi). Also you reduced them to 64 colours and into PNG-8 instead of PNG-24 - you physically altered dimensions not dpi.
With the files: dircopy.jpg , str60.jpg and ws60p.jpg they are all (400 x 300) and (107 dpi).
I think you have got yourself completely confused over DPI and colour depth and the fact that in image editors; they may handle re-sampling/sizing differently.
It sounds like your Image Editor is a little peculiar. I suspect when it mentioned CompuServe what it actually did was convert the image into a GIF or at the very least reduce it to 256-color. That would explain why you had those 2 freaky results (and only 64 differing colours) on the last 2 PNG files.
It's horses for courses, and PNG-24 would have a hell of a time trying to compete with JPG on a complex colour photo for file-size even if the JPG was saved at 100% quality. You've probably seen this: http://www.sitepoint.com/forums/showpost.php?p=4518703&postcount=23 and how I managed to get PNG to beat JPG on a like-for-like photo (without colour reduction).
Although without a doubt PNG-24/32 is the superior format for quality when using RGB images.
I hope some of this makes sense.