It seems everybody can use right-click on layers, and "save as" to export to .kml. I only get the opportunity to save as "GTiff".
What am I missing out here?
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I think your problem is you're trying to export a raster (tif) file to KML which, as far as I know, can only be done through plugins/scripts etc. Go to "Plugins"->"Manage and Install Plugins" and search for KMZ to see any plugin options, also see http://www.gdal.org/gdal2tiles.html to see how to use gdal to create a KML from a geotiff.
I recommend using "GEarthView" plugin. You just need to pick the layer You want to display and click plugin icon. This will launch GEarth with required layer displayed on a basemap.
At least it works in my case. I know that this is not straightforward, simple walkaround but it works :)