There are digital SSTV modes, check out EasyPal.
The problem with digital modes in general is they tend to be very clear right up until they are not, then they fall completely apart. Sometimes analog modes can provide better reception in fringe areas, areas where you would have no decode in digital modes you can at least have something with analog.
Of course, you can have digital modes that are very good in weak signal application, working at or below the noise floor, but those tend to be very low data rate modes, not something that works well with SSTV or information containing a lot of data, as an image does. You absolutely could make an image mode that works better at lower SNRs, but it would require you to take longer to send each frame.
T!