Jquery ui effects filename have changed. For example change

footer|combine_script require='jquery.effects.blind'

to

footer|combine_script require='jquery.ui.effect-blind'

We had backward naming compatibility for 2.5 but not for 2.6.

the file admin/themes/default/uplodify.jgrowl.css was moved to themes/default/js/plugins/jquery.jgrowl.css

use this line to include the file in your template :

{combine_css path="themes/default/js/plugins/jquery.jgrowl.css"}

no changes for the javascript file

Following http://piwigo.org/doc/doku.php?id=dev:changes_in_2.5#add_action_buttons
http://piwigo.org/dev/changeset/23263

Themes have to handle two new template vars : $PLUGIN_INDEX_BUTTONS and $PLUGIN_PICTURE_BUTTONS
Plugins creators SHOULD NOT add <li> or <div> or <span> around their buttons, it's now handled by the theme.

TokenInput CSS was moved from admin/themes/default/style.css to themes/default/js/plugins/jquery.tokeninput.css. Needs to be explicitly imported with

{combine_css path="themes/default/js/plugins/jquery.tokeninput.css"}

.

allowCreation parameter was renamed into allowFreeTagging