ImageTake

ImageTake[image, n] gives an image consisting of the first n rows.

ImageTake[image, {row1, row2}, {col1, col2}] gives a subimage spanning the specified rows and columns.

Examples

Take first 100 rows:

img = ExampleData[{"TestImage", "Lena"}];
ImageTake[img, 100]

Extract a region:

ImageTake[img, {100, 200}, {150, 300}]

Please visit the official Wolfram Language Reference for more details.