Depending on the image, a possibility is to check out the colour channels. Choose the one giving the best contrast between the background and the object you want to outline. Copy that to create a new alpha channel. Boost the contrast using levels and fill in relevant areas with black/white. Once happy with the result, copy this again to create a 2nd alpha channel. Use filter / other / max / min to increase the masked/selection area. You can then difference these two alpha channels to create your outline directly or use them for stroking/stroking path purposes. Hope this complements other method - use whatever gives the best? results - creating spare alphas also gives you the chance to experiment with other effects like blurs/grads etc If originally in RGB, and having trouble, try CMYK or LAB to see if they offer any other options.