Image with specific frequencies I have created a frequency domain image filter and would like to test it against some images where it is known that the frequencies only occur within a given band. Trouble is, I don't know how to go about creating such images.

I have though about making an image that consists of intensities varying about the x and y axes as a function of sine waves of frequencies within the band. I don't know if this would be good enough though. Another method (don't know about the practicality of this) might be to define a matrix in the Fourier domain and then do an iDFT. Does anyone have any thoughts on the best way to proceed?
Your second option is the most common: choose a frequency of interest, set the corresponding coefficient in the Fourier domain then take the inverse transform.
You have to pay attention to the correct scaling of the non-zero coefficient and to the particular FFT coefficient arrangement (continuous frequency in the middle or in one corner of the Fourier matrix).