How often do you give up waiting for a web page to open? The heavy use of images can be part of the problem.
Visitors to your website don’t care what format your images are in, they only care that they are viewable and downloaded in a timely fashion (even on slower connections). To accomplish this, images should be optimized for both speed and usability.
Firstly the choice of image type needs to be considered and by this I mean when to use a JPEG and when to use a PNG.
Here is a short explanation of the reasons why you would use different formats:
JPEG format is a lossy compressed file format. This makes it useful for storing photographs at a smaller size. JPEG is a common choice for use on the Web because it is compressed. For storing line drawings, text, and iconic graphics at a smaller file size, PNG's are a better choice because they are lossless.
JPEGs are for photographs and realistic images. PNGs are for line art, text-heavy images, and images with few colours.
The following link shows very good examples with explanations. http://lifehacker.com/learn-when-to-use-jpeg-gif-or-png-with-this-graphic-1669336151
Lossy or Lossless compression?
JPEG (Joint Photographic Experts Group) PNG (Portable Network Graphics)
Lossless compression means that as the file size is compressed, the picture quality remains the same - it does not get worse. Also, the file can be decompressed to its original quality.
Lossy compression permanently removes data.
You don't need to worry about the technical reasons why designers use different types of images in your website but having some knowledge will help you understand one of the reasons why your website may be a tad slow in loading.
Keep an eye on your SEO reports for individual pages and look at upload speeds on a regular basis, chat to your designer about reducing the size of images. It all helps to make a better, faster experience for your audience.
(includes extracts sourced from the web)