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"<%= RadImageGallery1.ClientID %>"
Adding a NavigateUrl to each image doesn't seem to be supported at this moment,
Here is a link to the feedback portal where you can track the progress of the bug and vote to increase its priority.
You could also develop your own custom control that fetches images, similar to our NewsRotator widget sample;
I hope the above information helps. Please let me know if you have any additional questions or concerns. In the case, please share how and where you have implemented this.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Sitefinity CMS Ideas&Feedback Portal
and vote to affect the priority of the items