Disadvantages of using a photo as a logo

I am considering using a small picture as my logo. The picture is of the Earth shaped into a light bulb. What could be the disadvantages of not using a drawing as a logo?

If it is an image, it becomes really tough to display in a lot of different formats/media. For instance, you'll lose a lot of quality if you have to print in grayscale, and a whole lot more if you plan on printing in just black and white. And yes, there will be times that this is needed. Also, consider people who still have those old-fashioned 16 color monitors (yes some people still have them).

Also, photos tend to not be very scalable. If you need to make it really small, it ends up being a little pile of pixels. And if you want to make it really large, your image either becomes really pixelated, or you have to come up with a way to add detail to the image. (Design in hi-res, I guess....)

I'd suggest going with a much simpler vector-based logo that looks good in just black and white--essentially, just as a outline or shape. (Words count as a shape too, if that's the way you want to go.) Ideally, people will be able to recognize it from just a silhouette. Think about logos like Apple, Microsoft, McDonald's, Pepsi, etc. From there, you can add detail on top of that (colors, shadows, glows, shines, whatever) in places where the extra details look good. In most cases, you're best off with a logo that works purely as a shape.

think of your logo on business cards, tradeshow booth, banners, signs, brochures, magazine ads, t-shirts. How good will a photo look?

For one thing, a proper logo design should be recognizable in a single-color context. There will be times when you'll need your logo to appear in pure black and white, for example. In your case, the fact that it's the earth would be lost if it were just a black light bulb.