Don't do that!

Convert your scale mode for both the form and picture box to 3-Pixels. Twips are bloody gay. There is no visible change except for every 15 twips, which = 1 pixel. Why have a gay scale that doesn't do anything?

Anyway. Do that ^ and then add the following code to your MouseDown/DragDrop event or whatever (not Mouse_Click - it doesn't pass the X/Y parameters):

X = X - (X Mod 32)
Y = Y - (Y Mod 32)

This will snap the mouse position (X,Y) to the top-left corner of the grid square you click in. (I think this is what you want... am I right?)
Note : This is for a grid of 32x32 pixel squares. Change the 32 as you wish.