First, validate your internet connection, and look through iPhoto's Preferences, there may be something there. All the obvious culprits. Since this is on three machines, see if the problem repeats itself. You might also try checking the Apple Support forums as well, this is a pretty new product and there's too many unknowns right now.
Separately, you don't need to both attach the screen shot and then IMG it. If you do use hosted images, please use the TIMG tag instead.