You may have found a proper solution to this in the meanwhile, but should that not be the case, read on for my easy solution to this.
I experience what I believed was exactly the same problem described here; attempting to upload pictures to your Photo Stream on iCloud from a PC w/ Windows 7 produces no result. What I first did was simply pasting pictures directly into the Photo Stream system folder, expecting that to initiate a synchronization.
That's not the right approach to upload pictures to your Photo Stream. Instead, the pictures should be pasted into the /Uploads subfolder underneath the Photo Stream folder! Go to your iClound Control Panel and click the [Options] button next to the Photo Stream item in the list. In the dialog being shown, notice the location of your Photo Stream folder. Navigate there, select the /Uploads subfolder and paste the pictures you want to upload.
When I did that, they began uploading almost immediately and a few seconds later, they showed nicely on both my iPad 4 and iPhone 5. :-)