It depends on the symbology of the image. If it is a raster image where colour represents altitude, this should be relatively straight forward. However, if it is a topographic map with contour lines, it will require much more work and probably the assistance of GIS software.

Can you upload a sample image and a reference of what you are expecting as your output?

Following up with what Mr. Barrett said, if the image you are working with is say for example a gray-scale DICOM, then when Matlab loads the image it is stored in a matrix, where each element of the matrix represents the gray-scale color for a pixel. You could treat this matrix like a surface if the image color does in fact represent altitude. To visualize, load an image to a matrix, then  plot using surf(imagematrix);

thanks @barrett and anderton