Because if the site is hosting pictures at all, it is hosting a lot of them, and their server probably can't handle thousands and thousands of high resolution pictures as high quality pictures take up a lot of space.
Bitmaps are memory hungry. In some cases a 700k picture in bmp will be 125 in jpeg and smaller in other formats. Remember, as a webpage downloads it tries to leave picture info in a temp file for reuse and to save the whole thing having to come down again. That is why to optimise the download speed smaller efficient picture formats are used.
swf is vector format, that means image can be re sized without loss of quality, but if you place bitmap or raster image in swf file when resized (enlarged) image will appear blurry.
Vector and bitmap are both image files. Bitmapped images are images that are stored on a pixel by pixel basis and because of this, when you enlarge the image it can appear blocky. A vector image is constructed from dots, lines, shapes, etc. Each part has a particular position within the image with it's own dimensions. Because vector images are constructed using images, they can be enlarged without loss of image quality. Vector images do not get the blocky appearance of an enlarged bitmap image.
Bitmap image is raster or pixel based image. Bitmap image is made up of pixels, every pixel in grid have its own position and color. Pixels are mapped to the pixel grid, that's why its name is Bitmap.
If you are referring to a bitmap image, it can be opened in Microsoft Paint, Adobe Photoshop, and GIMP. Most any image or photo program should be able to open a bitmap image.
Vector and bitmap are both image files. Bitmapped images are images that are stored on a pixel by pixel basis and because of this, when you enlarge the image it can appear blocky. A vector image is constructed from dots, lines, shapes, etc. Each part has a particular position within the image with it's own dimensions. Because vector images are constructed using images, they can be enlarged without loss of image quality. Vector images do not get the blocky appearance of an enlarged bitmap image.
bmp
A bitmap image doesn't necessarily have a specified height and width. It can be changed by pressing (Ctrl+E), or the long way; Image > Attributes
A bitmap is a series of bits which represents a rasterized graphic image, each pixel being represented as a group of bits.
A bitmap graphic is also known as a raster image.
a bitmap
A bitmap image uses solid color pixels, which can be identified and recolored easily. A lossy compression image consists of shaded colors which cannot be easily exchanged.
Photoshop can not save vector image but you can work with vector shapes inside Photoshop. There are many similarities, you can add elements to image like type, shape, change color... big difference is that you can not enlarge bitmap image without losing quality.