iir(filename,f,vara​rgin)

IIR Increases the size of an image by interpolation
B= IIR(inputfile,f) returns the image stored in file 'inputfile' with resolution increased by factor f in both dimensions. 'filename' must be a valid graphic file (jpg, gif, tiff, etc.). It can be grayscale or color.
Parameter 'f' is the size increase ratio, so to increase by 50%
use f= 1.5, to double size (in each dimension) use f= 2.
Additional parameters:
B= IIR(A,f,'Display','off') eliminates display of both images, the original and the modified. Deafult 'on'
B= IIR(A,f,'Method',method) Allows to choose between five methods of interpolation: linear, spline, pchip, cubic or v5cubic. 'method' must be a string character. Default 'linear'

Example:
B= iir('myimage.jpg',2);

The screenshot shows the effect of increasing resolution by 3. Original size: 600x402. After: 2400x1608.
(I took the photo myself. No copyright problems).