Bitmaps or raster graphics are 2D images that are collections of pixels painted of different color shades. Together, thousands or millions of these pixels are able to represent an image or color picture to the human eye.
Bitmaps are different from vector images like SVG, in that their pixel resolution is defined at their creation. This means that a picture stored as a bitmap cannot be made bigger or smaller without losing visual details.
Bitmap images are usually compressed to reduce their file size. This results in common file types such as PNG (Portable Network Graphics), TIFF (Tagged Image File Format), and JPEG (Joint Photographic Experts Group), which all use different types of compression.
FreeCAD can import bitmap images using the Image Workbench (version 0.20 and below), or using the Std Import tool (version 0.21 and above). Other workbenches can also insert bitmaps, like the TechDraw Image tool from the TechDraw Workbench.