I asked Google about the first gd-png error, and found a helpful page where they encountered this problem after upgrading from PHP 4 to PHP 5. Indeed, both MAMP and Dreamhost are running version 5 for me (eg, 5.2.3 in MAMP 1.7).
There's a partial workaround. Go to Admin Options > Image Display and change "Full image quality" from a value between 1 and 100 to a value between 1 and 10, eg, 8. After that, full-sized png's load fine. This disadvantage, though, is that full-sized jpg's look horrible. Guess the value for png needs to be divided by 10 in the source?