Great, you've decided to upgrade your site to Joomla version 3. For the galleries created with the extension Ozio Gallery there are no problems, just follow these guidelines.
Your images are all on Google Plus, so you do not risk losing them during the update process and they will not load the backup of your site (that we always advise to do before upgrading).
Ozio Gallery has two versions, one compatible with Joomla 2.5 and a different one compatible with Joomla 3. Then after doing the backup your full site and before proceeding with the upgrade to Joomla 3 you must uninstall the extension Ozio Gallery from your Joomla 2.5 site.

1) Go to Extensions > Extension Manager > click on Manage and insert the word ozio in the search field. Select 3 results:
Button - Ozio Gallery
Content - Oziogallery 3
Ozio Gallery 3
and then click on "Uninstall"

2) When you are ready with your upgrade process to Joomla 3 go ahead...

3) Now that your site has been properly updated to Joomla 3 go to the Extension Manager and install the latest version of Ozio Gallery 3 for Joomla.

4) From menu management of Joomla 3 control panel access to all menu items that refer to the galleries created with the skin Fuerte or those of any list of galleries. Save them again and check that they are published. Also check the various parameters, you will certainly discover new ones that have been introduced in this new version.
(even if you were using some of the old flash skin these are no longer available)

5) Check on your site that all galleries are displayed correctly.

6) Discover the new Nano skin and other new features included in the version of Ozio Gallery for Joomla 3.