The second tip should actually improve the image appearance even at 100%.
Try to paste the following code to
http://pdfcrowd.com/#convert_by_input and check the resulting PDF. The second image is a higher resolution version of the first image.
<html>
<body>
<img src="http://dl.dropbox.com/u/9346438/misc/img275x313.png" />
<img src="http://dl.dropbox.com/u/9346438/misc/img550x626.png" width="275" width="313"/>
</body>
</html>