Stock photographs are used for all sorts of reasons, to illustrate moods, physical objects, ideas etc. they may be used to illustrate magazine articles, corporate reports, brochures etc. The most successful ones can be used in several situations. The Top 50 shots are eye-catching, mainly with bright colour, good exposure and composition and most of them have areas were the user can drop in text if they want to.
In the first of your photographs is not instantly obvious what is happening, yes I can see that it is a spider, but what is it doing? I had to look twice to sort out what was going on and if anyone has to do that while trawling through hundreds of images then it will not be selected. It may be a perfectly good image of a spider with its prey but for a successful stock photograph it has to be instantly recognisable as such.
The second is better in that it is instantly recognisable. However, many stock photos used for publication will be used on a white or light textured media. Your black background, while it may be an effective way of isolating the subject, may limit the uses to which it can be put; it may be the preferred method of illustration for a technical work but not so good for the more general market. Try this one with a white background, even if details of the web are lost it may make the image more readily usable. Also, its looks as though the image is a slightly blurred, you may need to improve the depth of field to ensure that the entire spider is sharp.
The third one is just a photo of a dog, nothing particularly makes it stand out from hundreds of others. You say that you do not know why the photos from the Top 50 have been used but people have paid to use them, possibly several times over for some of them. Why would anyone want to pay for your photo of the dog and where could it be used?
If you want to improve your chances of success study your market. If you want to go for general stock photography then look at company reports, brochures etc where the images are used. If you want to specialise then find out how the subject should be presented; you can always try something different but also include the standard presentation, you will soon find out which is selling.
Imagine that you are trying to select an image. You have hundreds to choose from and a limited time to do it so you would not bother with anything that had the slightest hint of being out of focus, poorly exposed, badly composed or that needed a second look to determine what the image is showing. You have to make your images the ones that immediately leap out and grab the attention. Don't supply images that need to be manipulated by the user, most of them will not be interested, they are looking for something that is ready for use.
