The image sizes you can select from at admin/settings/picasa are not up to date. There is a newer list of allowed sizes for the imgmax parameter at
http://code.google.com/apis/picasaweb/docs/2.0/reference.html#Parameters:

94, 110, 128, 200, 220, 288, 320, 400, 512, 576, 640, 720, 800, 912, 1024, 1152, 1280, 1440, 1600

Additionally, the "original size" does not seem to work anymore (gives a 404). Currently it seems to use imgmax=max. However, according to the documentation of Picasa web albums, there is only a dimension "d" to get the original size.