One thing to look for in a good bottle image are smooth edges. This becomes important when the bottle needs to be separated from its background.
Photoshop has a number of tools that allow the graphic artist to select (outline) a bottle. Once outlined the background can be removed (made transparent) or replaced. But the quick and dirty tools like the Magic Wand, Quick Select and Lasso tools generally leave jagged edges. For a smooth line, one must use the 'Pen Tool' to make the selection. Of course this takes more time and requires practice to become proficient. But if you look at the examples, I think you will see that using the Pen Tool is worth the effort.
So, when examining a bottle image, make sure to zoom in to see if the line is smooth. And if it is smooth, double check to make sure the bottle has already been isolated from its background. If your photo's background has not been removed by the photographer, you might have to pay additional money to a graphic artist to do the work in the future.
Smooth Edge Good
Jagged Edge Bad
