Can you recommend me a fast & easy service to generate thumbnail images?

I usually take advantage of the Unsplash images collection to get free images for my projects. However, the image size is usually huge, so the images take a long time to load every time the user opens the website?

Can you recommend me a program or service to resize large images into a size that is website-friendly? It would be awesome to find a site where you can download Unsplash images in a lower resolution that is better for the user experience.

Thanks in advance!