Finding amplitudes from FFT data of a 2D image
If you can adjust the orientation of your image so that the sinusoid pattern runs vertically to the image, then you can obtain a vertical slice, perhaps at the middle of the image and use that as a 1D waveform for further analysis.

If the waves can come out of your experiment in any orientation you basically have two options:

1) Estimate the angle of rotation ($a$), apply a rotation by $-a$ and then select a slice of the image (just as in the introduction). To estimate the angle of rotation you can look at the output of the Hough Transform which will give you strong "spots" at a given angle for each line (or line-looking) form within your image. That would be the elongated folds of the wrinkles in your image. For more information please see: http://en.wikipedia.org/wiki/Hough_transform

2) Do a straightforward 2D Fourier Transform to recover both orientation and amplitude spectra (at least for the example you provide). The sinusoidal-like form will appear as a linear feature of some orientation in your FFT spectrum which you could pick up with a simple thresholding operation. For more information please see http://www.robots.ox.ac.uk/~az/lectures/ia/lect2.pdf

Hope this helps.