Image sizes should be from around 40kb to around 550kb. We suggest small images to be around 100kb and 350kb for large images.
You should try to not upload images that are over 1MB (~1000KB).
This is why your images should have a small file size:
- You use more server space and you can one day reach your allocated space's limit and you'd wonder why your website is not loading well anymore.
- You'll use more bandwidth in your hosting, Internet server at home or work, and your visitors'.
- Your visitors will notice that your website is taking time to load, or at least when opening images. It will make the experience worse and visitors don't like to spend more than 3-4 seconds waiting for a page to load. They might end up leaving.
It's better to take care of this early on rather than have to compress your images when you have fully set up your website. It's never too late to start, however.
What you should do first, is to make sure that you have all your images on your computer and not just on your website. Then, delete all the ones you have added in your website. To do so, open any page where you can add images, then, when the image manager is open, select them all and delete. If you have various folders, go in each folder and delete your images.
On your computer, compress the images. You can use programmes such as Light Image Resizer: https://www.obviousidea.com/windows-software/light-image-resizer/
Or, if you have Photoshop or other image editors, you can compress your images using File > Save for Web (in Photoshop).
If you can't use those above, then you can reduce the resolution of each image using programmes such as Windows Paint or Mac Photos (or Paintbrush). For your banners (full screen slideshows), images should have a minimum width of 2000px. For smaller images, you can have landscape images at 900px wide, and 600px wide if they're portrait images.