I agree with both of you mostly. Actually, with the plants, I didn't mean animation as much as just using plugins and effects.
Once a month sounds good. Originally I was thinking that someone could take most of the week to make something so they wouldn't have to cram in a lot of work all at once, but once a month still might be better. I'm sure most people are busy with their own projects too.
As for using ripped sprites, I'm not exactly in favor of that because no one can legally use ripped sprites even for non-commerical stuff.