I ran across a site that made me laugh. The site presents 500 (500!) alternatives to Picnik. Driven souls at more of it have create bots to search the web looking for Picnik replacements. I've settled in happily with iPiccy, but if you're still searching, you might want to check out the more of it list.
The interesting thing about the list is that it includes a line at the bottom of each entry that allows you to quickly evaluate each entry. Here's a sample of the bottom line.