A random word generator -- as long as you have the right to junk words for being non-genre-appropriate -- I've found very useful. It opens up a lot of possibilities. (The ones that are homographs are the most fun, but the whole point of randomness is to break things up into new configurations.) Indeed, they work best to kick loose ideas about what happens next, not
Pictures can also work. But pictures are rife with details. It's not only more likely to be genre-specific, it binds the setting more. And it limits the plot events. Useful for coming up with future possibilities to aim for more than moving at the moment.
I've also used Rory's Story Cubes. They come in sets of nine and sets of three -- different types -- and while they are genre specific, I think the three sets are more useful to jog your ideas loose and get you rolling. Of course, you could just choose what you liked when you rolled the nine, and skip the rest, but that tends their usefulness in making things go oddly. (And they do sagely refuse to explain what the symbols are meant. Only tell you they can be symbolic as well as literal.)