Based on your reaction type, you should aim to use an image with the following size (or ratio). Don't worry if your image is a different size, we will automatically resize the image for you.
(all dimensions in px)
Lightbox: 480 x 289
Side Bar (Left / Right): 350 x 207
Notification (Left / Right): 350 x 225
Tooltip: 120 x 133
Lightbox / 2-Step Lightbox: 337 x 200 (portrait) / 601 x 250 (landscape)
Note: Images will scale according to device orientation.