ICO is a format (full name - Icon File) of bitmap images and refers to system files. Developed by Microsoft and designed to store icons in its operating system. ICO files, as a rule, store icon images - modest in size, with a small resolution (from 16x16 to 64x64 pixels), with different color depths (from 16 colors). In the graphical interface of various operating systems, ICO format files are used to display files, programs and folders (primarily in Windows) or as an icon for a specific website.
BMP is a graphic format that allows you to store bitmaps as a bitmap based on single-layer rasters. It was developed by Microsoft. She also implemented it in the Windows operating system. The BMP format (English Bitmap Picture) is used in the work of many programs, since its support is integrated into Windows and OS / 2. Files of this format can have the extensions .bmp, .dib or .rle. The BMP graphics format has often changed. From the very beginning, the image structure was built by combining small squares, that is, pixels, each of which is capable of containing data about what color depth is. It happens to be 1, 2, 4, 8, 16, 24, 32, 48 bits per pixel. The size of the final image depends on the bitness. The BMP file format still looks like a monster, since it has large sizes. After all, the image is uncompressed, which can not be said, for example, about the JPG format. Any Windows operating system has the means to open such images. The main editing tool has always been Paint - the native Microsoft program that is in every version of Windows. In BMP, images are stored in their original form, but compression algorithms can also be used. Let's say BMP provides quality support for RLE compression.