DCT of an Image

I want to perform DCT on an image. I divided 600*800 image into 8*8 blocks using mat2cell. Now I do not know how to access individual cell. Also how do I use dct2 to the cells which have been created. Part of code:

RowBlock = 8;

ColumnBlock = 8;

ROW = [RowBlock * ones(1, 75), 0];

COLUMN = [ColumnBlock * ones(1, 100), 0];

FinalImage = mat2cell(Yimage, ROW, COLUMN)

Thank You!