I have a VERY similar problem - my difference is that I had ATT do an upgrade on my Galaxy SII to Ice Cream Sandwich and I now can't see any photos that were on the phone previously. (I was getting a loop in the touch wiz interface and couldn't access the phone the lame ATT tech at their service center said all he could do was do the upgrade -- I figured out later that the Keis software could have saved my photos.)Welcome. I feel badly about your situation, must be a terrible feeling.
Don't worry too much, the images are probably not gone or deleted. The pictures are probably still there
Normally I would have a few suggestions to try, however since you are dealing with extremely delicate data and I'm not physically there to help I'm not going to suggest any fixes to try for fear of overwriting the data.
Just because they are not showing in the gallery does not mean they are gone. Sometimes the Gallery application can have hidden folders or malfunction and not display things correctly. It is a side issue, your main focus should be the image files.
Quickpix could possibly be showing the files from a new file directory, or there could be thumbnail files leftover that this application is finding and displaying for you. Another side issue of sorts.
I would recommend highly not taking any more pictures. If in fact they were deleted, there is a slight chance that taking a new pictures or adding new data could overwrite the location of previous pictures which you are wanting to find and maybe recover.
The update could be a significant detail here, during an update they possibly could have been temporarily moved or cached to install the new software. Not sure where this takes place or where they would be stored (they normally get moved and put back), but just know that it is a function of some updates.
I also would not take it to Sprint. The chance of them losing your data forever is too risky in my opinion. You would be better off if you know a friend or acquaintance who knows quite a bit about phones and can investigate for you without modifying the current set up until it has been determined the pictures are found or were indeed deleted. If you find the images well then success! If you find no trace or find that they were deleted there are some recovery options out there.
For the future I recommend implementing a plan for scheduled physical back ups. And as a golden rule always always always do a back up before installing an update or modifying the phone.
Hope this all works out the easy way and you find the files.
Will the same techniques work for me? I'm about to attempt to clone the phone to a hard drive as a back up and have something I can dig around in, instead of the actual phone.
I also found some Computer Forensic sofware to clone the drive.
Any further advise is greatly appreciated.
JP

