This is a great tool -- thanks for sharing it and for building in native WUnderground support. I have a couple recommendations:
1. For me, the smaller of the two image sizes (Large) is still a bit too large. I would recommend--in addition to the Large and Extra Large sizes--that you consider including Medium and Small. In fact, I rolled my own (Medium is 25 percent of Extra Large and Small is 25 percent of Large.)
2. For WUnderground custom state support, please consider adding alertType1 - alertType5.
Dave