Is there any way to empahasize a particular area of a photo, such as in Flickr? e.g. where you drag and create a rectangle around something you want the viewer to see that they might not otherwise notice?
I suppose the watermark feature could be used to accomplish that, although it was not designed to, so there would be a few caveats; for example it could interfere with your normal watermarking if you use the feature already. However, you can have several different watermarks (albeit only one applied to an image at once) and you would require a 'Pro' account
I would recommend using an image editor (such as Photoshop) to alter the image as you see fit before uploading. If you save the original image as well (on your computer or on SmugMug), it would be easy removing the "effect" later on by simply replacing the image. It may not be what you want if you need the editing to be "soft" and online, but it would work and give you much more flexibility in the appearance of the effect than a online solution could ever aspire to do.