My name is***** am sorry to hear that you have this issue, I look forward to assisting you today with your question and providing the best answer possible.
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Try setting Picasa as default Photo Viewer application..
To set it:
1: Right Click a Picture >click Open with > click Choose Default Program >
2: Check mark this option "Always Use the Selected program to open this kind of file"
Now select Picasa App > Click OK
Now try to open another picture and see if it opens in picasa
Please update status
Wish you a good day...
I would like to know the status of this call.
Did my suggestion helped..
If you still have trouble, i will start a remote session and do the process for you