This of course will cause the browser to give you a nice little red x. All this would be solved if there weren't so many different graphic formats. Of course, each format is different, so the solution is to pay closer attention, and also to learn what each format is, and why they are used in different situations. The first thing that we need to understand is that there are 2 types of image compression, lossy, and lossless. Different image formats use different compression formats.
Lossless compression compresses an image without removing any data.
Lossy compression will discard some of the data so that it can be compressed more. Lossy images typically wouldn't have as many colors available. JPG, JPEG, or Joint Photographic Experts Group Format images have lossy compression. This is a common format used when saving photographs. Doesn't save line art very well.