There are several tools available to help you optimize your images. These tools will reduce the file size of an image while still allowing it to be displayed at full resolution on a user's screen when they click or tap on it. This prevents blurry-looking images, which can sometimes happen in responsive design if your image files are too large.
Photoshop is a very powerful tool, and there are several good tutorials out there about using it to make images lighter. However, it can be expensive, which makes it prohibitive for many people who want to optimize their website's images before uploading them. It also requires that you have some level of technical know-how in order to use the advanced features of it.
Photoshop is not your only option though. Alternatives do exist, and they are both free to use and fairly easy to learn.
One alternative is ImageResizer. This tool allows you to quickly resize images in bulk without needing Photoshop or any other software installed on your computer. It's fast, it works well, and it's free. It is a browser-based, which means that you simply upload your image files to the site and it gives back a URL of new compressed images for use on your website. It makes this process easier and faster, because it doesn't require Photoshop or any other software installed on your computer in order for you to use it! The only downside of using it is that if you upload huge files (e.g., images that are several megabytes in size), it will only return a smaller file instead of automatically resizing the image for you.
ImageCompressor is another great alternative that I recommend using if you need Photoshop but don't want to spend the money on purchasing it or learning more. It is Photoshop-compatible, so you can upload your original Photoshop files to the site and have them compress in seconds! This is great because Photoshop files tend to be very large, and it can take quite a while before the site compresses your images. The only downside that I've noticed about ImageCompressor is that sometimes when you compress an image in Photoshop (for example), there are different compression settings available for image quality. Photoshop remembers what setting you used last time you compressed the image, so you don't have to worry about it. ImageCompressor does not remember this though, which means that every time Photoshop saves a JPEG as "smallest," for example, it will automatically compress your image using those settings instead of letting you choose them yourself from within the interface.