Welcome to Wolfram! There are a few language problems in your notebook.
- MatrixForm is useful for looking at data in a notebook. But it should not be used for subsequent evaluations.
- Don't use "_" on in symbol names.
- Semicolons are your friend for large outputs.
That said, here are some inputs that do what you want (I think):
x2 = ImageData[Import["ExampleData/lena.tif"]];
Image[x2, ColorSpace -> "RGB"]
Below, you are trying to do some matrix evaluations with incompatible sized matrices. I'm not sure what you're trying to do here, so I'll leave that problem for someone else.
In[56]:= Dimensions@x2
Out[56]= {116, 150, 3}
dct1 = FourierDCTMatrix[8];
In[57]:= Dimensions@dct1
Out[57]= {8, 8}
