How can I split this image?

Are you talking about a bitmap image?
And since this is the Draw forum I assume you are working with Draw, not PhotoPaint?

There are several ways to do it, here's one rather simple:

Create two straight lines, one vertical, one horizontal.
Center them both to each other vertically and horizontally and combine them.
Center the combined "+" to the bitmap, again both vertically and horizontally.
With the "+" selected, shift click to select the bitmap.
Use Shaping > Trim.
This will split the bitmap into four pieces but they are combined so you must break them apart.

Note that this way to split the bitmap will in fact only hide the parts that you cut away.
This means that you have four copies of the original bitmap with different parts hidden.

If you truly want to get rid of the hidden parts, the Crop tool is useful.