Firefox Page Info feature makes downloading images in bulk from any website lot easy, you can even sort according to size or type. Now if you want to go ahead and want to filter according to particular range in size i.e. Image of particular size or between range with a minimum of Image size, you need the Image Picker Extension for Firefox.
When you want to download images from a website, all you need is right-click and choose Image Picker. It then lists all the images available on the page with a thumbnail preview. Now you can use filters to grab only PNG or JPEG or BPM or GIF files. You can also set size and dimension filters.
Once you are happy with the results, select the images and save them to location you want. Downloading is fast as images are copied from your Browser cache which should have already downloaded the image once. So make sure you let the page load completely to take full advantage of this extension.