How can I insert an image(jpg/tiff) to an existing GIF in Preview?
Preview is not an image editor.  It's a viewer.

To create images, you need something that allows you to do this.  You can get paid applications to do this and there are FOSS (Free Open Source) tools available.  Some of the more popular ones are:


Adobe Illustrator (Commercial illustration software)
Adobe Photoshop (Commerical photo editing software)
GIMP (Open Source Alternative to Photoshop)
Pinta (Like the award winning Paint.NET for Windows)
Tux Paint (Award winning paint program for kids)


There are many, many more available on the App store and online.  

And if actual photo editing software isn't your cup of tea, Word Processing programs like Microsoft Word has rudimentary image editing built in.