I entered this photo in the photo.stackexchange.com picture of the week contest last week, and quite unexpectedly, it won. I say "unexpectedly" because I considered this a fairly unremarkable photo, but there were a few things that helped it out in this contest, I think.
First, it's a very simple subject, so it's well-suited to small-scale use in the banner size of 375x210. The photo you see here, in fact, is scaled larger than 100% by Lightroom, so the photo I submitted for the contest is actually very close to a 100% crop, and it's pretty sharp at that size (below).
The other thing this photo has going for it is a really consistent color theme (green, obviously) with a contrasting color in the subject that stands out from the background without clashing. Again, when viewed at the smaller size, it's easy to see the subject of the photo, and the green background isn't so complicated that it competes for attention.
By the way, if you look carefully at the eye in this photo, you'll see a catchlight from my flash - this frog was somewhere around 20 feet away, and I hit him with a fairly powerful strobe in order to get the yellow in his eye to pop like that. Since this shot is all about that one big eye keeping a watch on me, the lighting from the strobe was key to getting these colors to show up the way they did.
If you like this photo, be sure to check out others from the same hike.