Here is an example of what you can do:
%open an image
A = imread('coins.png');
%run roi tool, where you can define your polygon
BW = roipoly(A);
%extract image pixels in the roi
S = regionprops(BW,'ConvexImage','BoundingBox');
r = S.BoundingBox;
Q = A( round(r(2)) + (1:r(4)) , round(r(1)) + (1:r(3)) );
%display cropped image
im = imagesc(Q);
im.AlphaData = S.ConvexImage;
axis image; colormap gray;
In the roipoly window track your points manually to define a polygon, as in the picture:
Then, right-click and "Create Mask", you will get your cropped image:
