The details on what you are doing are a bit slight.
You should be able to see the imported tif by using Show[Picture]
If by "Do'in some image processing" you mean that you are changing the values of
various parts of the symbol Picture, you should be able to see the results of those
changes also using Show.
I am not sure what you mean by "I´ve tried to make a densityplot but it has given back
only the "graphic" which was made from densityplot. " Do you mean that you saw only
the output, "-Graphics-". or that you saw the density plot and it was some how not
what you wanted.
If you have V4.0 or later you can use the Export command to export the file as tif, e.g.
Export["filename.tif", Picture, "Tiff"]
In V3.x Expor is not available but Display can do the conversion:
Display["filename.tif", Picture, "TIFF"]
This assumes that Picture is a complete graphic expression, not just a list of numbers.
One way to test that is to evaluate Show[Picture], if you get a graphic then the expression
is okay. If you look at InputForm[Picture] just after you Import the Tif, you will see
the contents of the expression.