That's inefficient and confusing. Just take the histogram again when you want it later, or if you want to save the histogram itself, just save the histogram with save(). Don't save a displayed figure, with all its background, axes tick marks, etc. and then try to get back to the original image from a figure - it just doesn't make sense.
% Save the cropped image
imwrite( croppedImageArray, fullFileName);
% or save the histogram
[pixelCounts grayLevels] = imhist(croppedImageArray);
save(fullMATFileName, 'pixelCounts', 'grayLevels');
