How can I optimize images on my site?

Optimize Images
1. Reduce file size
2. File Name
3. Url source image
4. Alt text & Title text
5. Caption

Use Tinypng to compress the size of images before uploading. Then use Wp-Smush or something other plugin to do that. After that you can use https://developers.google.com/speed/pagespeed/insights/ and it will help you download the compressed images and you can upload them.

Not only will this cause visitors to bounce, but in time, Google will notice as well, and this can affect your SEO.

To Optimize Images on sites:
1. Reduce file size
2. File Name
3. Url source image
4. Alt text & Title text
5. Caption.....

Well .... amazing answers above which are very well described. I only want to say that .. images plays an important role in your branding. So image optimization is important thing to do.

After image optimization .. you can also use your images in social media and image submission websites.

1. Name your images descriptively
2. Optimize Alt tag
3. Reduce the file size
4. Use image site maps
5. Url source image..

• Image size: The minimum image size is 320 pixels and the maximum is 1280 pixels. Images too small to be rated will be poor. Larger images will also have difficulty ranking. Because large images only appear if the user has the option "big picture".
• Proportional format: For best results you should use normal image formats such as: Square at 4: 3 or 16:10.
• File name: It is a very important factor if you want to optimize your images successfully. Most photos can only be found if they contain keywords or phrases in the filename (eg "ky-thuat-cat-loi-trum.jpg"). This gives you the ability to rank with the keyword "cutting profit" or "cutting bosses" or "bosses" and so on. However, you should not spam file names: should use filename from 4 -6 words. And of course the name should be a kind of description of what can be seen on the image.
• Copyright Notices: This does not involve image optimization. However, you should include copyright notices in the images before uploading them. (To put it bluntly, it's a good idea to tag your image so others do not use your photo.)
• Image type: JPEG, GIF or PNG? I recommend to format JPEG or PNG images. According to my observation: the images in the JPEG format are ranked better. The use of PNG images is similar, but what is important to me is the speed of image loading is fast?
• Uniqueness: Apparently Google loves diversity, so be sure to try to create unique photos and check that your photo can be TOP.

There is indeed a minimum and maximum for pictures.
But nowadays your sites should be "responsive", you have different sizes of windows...
Mainly those on small tablets / big tablets / laptops and pc with bigger screens ( not too much blanc left / right of screen )

It is simple to optimize the images for your website
1. Don't use large size images, compress them before uploading
2. Use alt attribute to tell the SE Spiders about the images
3. Use Keywords
4. Caption (Description)
5. Avoid special characters in image text(alt attribute)

it is very simple to optimize an image for the site.
Use ALT tag to optimize the images for the site. Pick the right name related to your image content. Keep the right name for the image URL.
Reduce file size for faster loading.Add a caption for easier scanning of the page.create an image sitemap
Google encourages you to use image sitemaps to give their search engine more information about the images available on your website.
The image should reflect the topic of the post or have illustrative purposes within the article, blog, etc.

When you post select a Focus Keyword. Then try to use that Focus keyword in your File name , alternative text and in the Description.