This post is based on an old discussion about the same problem: Picasa's screensaver keeps rotating photos as it displays them in its screensaver. When the said photos are viewed in Picasa software by simply double-clicking them, the image is displayed properly; however, when the Google screensaver's launched, images are rotated.
I use ACDSee mostly to view photos as well as to rotate them. Apparently, Picasa is incapable to correctly interpret the EXIF data in regards to an image orientation. Thus, a conflict's created. A photo may look properly rotated in both ACDSee, Microsoft Viewer, Picasa program itself, but not in the Picasa's screensaver. Therefore, it's important to see the image real (but hidden) orientation before it can be corrected. None of the aforementioned programs can do that, at least not easily or intuitively.
A simple solution to this is to obtain a software that will display a photo's orientation as it truly is. One such program is IrfanView (http://www.irfanview.com/). When a photo opens up in IrfanView, the way it looks and positioned interpreted correctly by reading its EXIF data. One can then use the software's function called 'JPEG lossless rotation (plugin)' to introduce the necessary corrections. No saving is needed at the end. Having Picasa program open at the same time ensures that it updates the corrected photo automatically, and if such photo is part of Picasa's screensaver, it'll be displayed properly from that point on.
Hope this information helps to all those who still ask the above question all over the net with no clear solutions.