We need up to five screenshots for the web browser stores (Firefox and Chrome). For example, this is one way they're displayed in Chrome.
Firefox (1 screenshot):
PNG and JPG supported, PNG recommended. The maximum and recommended size for screenshots is 2400x1800 pixels. Having screenshots in your product page greatly increases its chances of being installed and being featured in the homepage.
Chrome (up to 5 screenshots):
Extensions, applications and themes require at least one screenshot or YouTube link.
Provide preferably 4 or 5 screenshots of your app (up to a maximum of 5). If your app supports multiple locales, you can provide locale-specific screenshots. Your screenshot should have square corners and no padding (full bleed).
We prefer screenshots to be 1280x800 pixels in size, as they will be used in future high dpi displays. Currently, the Chrome Web Store will downscale all screenshots to 640x400, so if your screenshots will not look good downscaled (eg. have a lot of text) or if 1280x800 is too big for your app (screenshot of low resolution game), we also support 640x400 pixels screenshots.