I am not very familiar with the specifics, but here is what little I know: A lot of image processing algorithms are developed in the academic world, so you may not have to worry about patents if you just replicate these methods. If these methods are older than 20 years, then there is no patent issue anyways. (You'd be surprised at the number of times some smart scientist coming up with a very clever solution, even though it was not practical to implement at that time.) Most of these algorithms can easily be expressed as mathematical equations and math is not patentable, so maybe a lot of these image processing algorithms are not patented to begin with.