I disagree with the approach of flattening, printing, or otherwise destroying information in the final-result file, because there can be a lot of learning value for the students in having the solution. The approach of manipulating the image in some way and attempting to detect that modification in the result could work, although it seems like a lot of effort.
Instead, could you require the students to submit the intermediate results as well? That way you have more evidence that the students actually performed the steps. Also, if there is any variation in the steps, it gives you more information about possible copying between students if all intermediate results are the same in addition to the final result.