CImageDecoder / *.png

That means there is some mismatch in argument types.
If you read it, it will actually tell you exactly what the problem is (though can be hard to interpret without good C++ knowledge)

Maybe because those strange const strings you have tried to declare.

A much better way to declare those would be with the _LIT macro:

_LIT(FileName,"map.png");
_LIT(Mime,"image/png");

Try that first...

Oh.. also, you should not have the & on the Fss...

oh, you shouldn't have an & on any of them.

The & in the function declaration just means it will take a reference to the object instead of doing a copy. (basic C++ stuff)

Try this:

RFs Fss;
Fss.Connect();
_LIT(Filename,"map.png");
_LIT8(Mime,"image/png");  //This needs to be an 8 bit descriptor for some reason...
CImageDecoder ImageDecoder = CImageDecoder::FileNewL( Fss, Filename, Mime, CImageDecoder::EOptionNone );

Also, if you are writing an application, you can get a reference to the current file server session (already opened by the framework) by calling

CCoeEnv::Static()->FsSession()

or.. if your code is within a CCoeControl class, you can get it even quicker by

iCoeEnv->FsSession()

Oh, didn't notice you didn't have the * there, ofcourse its needed
I think you might need to get a C++ book, and a basic symbian book and read through...
Your next step before this is proper symbian code is to add error and resource handling with cleanupstack and protect against leaves.

Your new error is a link error because you havn't included the library in the mmp file.
you need to add a line to it:

LIBRARY imageconversion.lib