I did not know that I can easily re-arrange images in G+ albums, but just now I found the way!

1. Go to "photos" on your g+ bar, it will open picasa albums. all your images are there.
1.a Load album by clicking on it
2. Click "Organize"
3. Re-arrange by drag and drop.. You can also set cover image in action drop-down over there.

See illustrated below.

Worth to share? :-)
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