When working with websites - several common image types are used
One common image type is .png and is currently the most popular template graphic file.

PNG stands for Portable Network Graphic and is intended as the replacement for the .gif format. PNGs like GIFS do offer transparency and use lossless compression but PNGs have the advantage of supporting millions of colours. However PNGs normally are of a larger file size than both GIFs and JPEGs - however you can optimise the file size.

PNGs are available at 8bit, 24bit or 32bit - with the increasing bit size meaning more colours rendered and 24bit and up supporting transparancy (PNG-8 is suitable for solid colours rather then more complex imagery)

Sites like tinypng.com/ offer the ability to optimise PNGs