Which image file format is best for preserving transparency for web use?

The PNG (Portable Network Graphics) format is best for preserving transparency for web use due to its support for a full range of color depths and its ability to handle alpha transparency. This means that PNG files can include varying degrees of transparency, allowing for smooth edges and complex shapes, which is particularly valuable for images like logos or graphics that need to blend seamlessly into different backgrounds.

In contrast, JPEG (Joint Photographic Experts Group) does not support transparency at all, making it unsuitable for images requiring transparent backgrounds. GIF (Graphics Interchange Format) does support transparency but only allows for one color to be fully transparent and does not accommodate varying levels of transparency, which limits its utility. BMP (Bitmap) files also do not support transparency and are primarily used for high-quality images rather than web graphics. Thus, the PNG format is the superior choice for web images when transparency is a critical factor.