I have a tiff image file and want to get a polygon with the extent of the tiff. Is there a direct way of doing this? Right now i have to convert the image to vector (with gdal_polygonize) and then dissolve the polygons...
Your question isn't clear. Do you want the 'rectangular' extents of the raster, or the extent of the features in the raster? For the rectangular extent, just use the "Tile Index" under Raster > Miscellanious > Tile Index. If you don't see the Raster menu, you may have to install or enable the GDALTools plugin.
And of course if you aren't tied to doing it in QGIS, you could just use GDAL directly. See gdaltileindex