I take screenshots alot, for various reasons, and I also download images from the tubes. I prefer this type of image to be filed in a small folder for easy drag-drop access. I do not want these images in iPhoto; thats where I keep family/personal photos organized.
So Hazel does this:
Filetype picture
if the filename begins with "Picture" then move to "/Photos/Screenshots/"
if the file is less than 1 mb move to "/Photos/Clips"
if the filesize is larger than 3 mb import to iPhoto
if the filename begins with DSC*, import to iPhoto
else move to "/Photos/Misc"
Growl
Although not exhaustive, it hasn't failed me yet.