Most issues with image uploader or form validation is related with scripts errors which are caused because of multiple declaration of jQuery library. Not every extension/template uses basic Joomla! scripts. Some include their own scripts.
This can be fixed quite easy, using free plugin you can download: DJ-JQueryMonster.
DJ-jQueryMonster plugin helps you rid of the conflicts with jQuery.
The plugin sets correct order of the scripts, removes all enabled jQuery/no-conflict instances and enables own jQuery and no-conflict mode helping to solve conflicts.
In settings you can choose jQuery version, jQuery UI and jQuery UI Theme - all from select lists (those come from Google's CDN) + option to provide custom version or URL for each.
You can also provide path to the script that should not be removed (if needed).
First download and install the plugin DJ-JQueryMonster then play with the settings and check if it helps in your case (there's no recipe unfortunately due to different extensions you may be using on your site).
If the issue still occurs please contact our support team.