I still like different birds, and am actively looking for an owl at present that would depict well, though haven't found one yet. It would likely only be the head, as an owl otherwise wouldn't show well at that size. Lots of things look great, then when reduced to around 20px in height, suddenly no longer look so good.
I personally find the majority of the images I've seen you try so far, quite difficult to work out what they are, simply because they are so small, they are out-lines, rather than solid shapes, and there are a lot of pale colours being used.
I think you need to be looking at solid shapes (silhouettes rather than out-lines) and using bold colours. Or perhaps start off with a paler shade, and build to a darker shade of the same colour across the 4 images - rather like the green bar. That way the image would become clearer as you develop through that rank.
The types of solid images I refer to, are those such as . . .
[DLMURL]http://www.istockphoto.com/stock-illustration-20647627-eagles.php?st=b30265a[/DLMURL]
[DLMURL]http://www.istockphoto.com/stock-illustration-2206004-assorted-birds-vector.php?st=099bcc0[/DLMURL]
[DLMURL]http://www.istockphoto.com/stock-illustration-21318027-birds-black-and-white-icon-set.php?st=b30265a[/DLMURL] I quite like the hatching egg in this one, as a starting point for new members, if you stick with the bird theme.
I don't know whether you are already dead-set on the bird idea, but as I was thinking about these images (particularly the thought of using silhouettes), I thought about the evolution of man, which is so often depicted in silhouette form. It is a great representation of growth, development, evolving, moving forwards etc. And is also easily identifiable with regards to the order in which they come. For example
[DLMURL]http://www.istockphoto.com/stock-illustration-18665958-evolution-of-the-runner-and-hiker.php[/DLMURL]
[DLMURL]http://www.istockphoto.com/stock-illustration-18636128-evolution-of-the-cyclist.php[/DLMURL]
However, as with the Dodo, I guess people might be offended by it?!
Just on a side note, I don't know how you resize images or what program you use, but if you use photoshop, selecting "Bicubic Sharper", when reducing image size, gives much better quality than standard resizing.