The .gif format is supported by default. You can get a full list of the supported formats ( and add your own if you want ) from Settings -> Advanced -> Libraries -> Images ( you can check the attached image ).
the Telerik team
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 Public Issue Tracking
system and vote to affect the priority of the items