Are all grayscale images of the size 256*256. Please clarify

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I am using a .png image for processing. I converted it to grayscale and the value shown for the image is 289x318 uint8.. When I used [r c d]= size(I), I got r= 289, c=318 and d=1. Does it mean that my image has been grayscaled or not? And what does size 256*256 mean? Does it signify the values inside the matrix?

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Thorsten
Thorsten on 27 Nov 2015
Edited: Thorsten on 27 Nov 2015
The size of your image is 289 x 318, as returned by the size command. Why do you think that the size is 256*256? A grayscale image can be of any size, only some of them are of size 256 x 256. The range of values are from 0 to 255, for a uint8 type image, so each pixel can take one from 256 different values. But that's the value of a pixel and has nothing to do with the size.
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Thorsten
Thorsten on 27 Nov 2015
Edited: Thorsten on 27 Nov 2015
You can crop the image
I = I(1:256, 1:256);
You can also resize the image using imresize, if it is square, or use a combination of resizing and cropping:
[r, c] = size(I);
I = imresize(I, 256/min(r,c));
I = I(1:256, 1:256);

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