The first three images on my watercolour site are the logo ones. To speed up page loading the logo is split into 3 separate images and all these have alt tags. In fact every image on the site does except for the nav bar.
Even with dropping down the rankings a little, which won't be for long, it hasn't hurt our visitor numbers. In fact the number of visitors is steadily rising all the time and we now have members from all corners of the globe. All members are actively encouraged to advertise the site where ever they can and this they do.
The percentage score I think is a very good idea because people will be more determined to make an effort in improving there source code. It's that competitive streak in us all.
Amateur Watercolour Community