I'm afraid that a search for plants with no images and for plants with no data might run into the hundreds of thousands. At this moment, there are 637,994 plants in the database (with more being added each day) and 249,061 images. We know that many plants have multiple images, but even if they didn't, that would still be a list of close to 400,000.
Here's a good way to start: Look at your own plant lists and notice the plants that have no images attached. Take pictures of all of your plants. Maybe you're conscientious enough to have done this already, but I have dozens of plants on my list with no images.
You could also "adopt a genus." Some members have done this and have filled in the data for each entry in the genus. This could be a good project for the cold and rainy season when you're stuck indoors.
