I am not sure how common this format but, does babylonjs support targa format, or does web browsers support this file format at all? Since babylonjs targets browser, apart from NullEngine I have come accross docs, it all comes to this.
Just come accross this post: Babylon.JS Sandbox now supporting DDS & Targa - Questions & Answers - HTML5 Game Devs Forum
Yes tga are supported at some extends.