Free image optimization tools

My site is loading super slow on mobile, and I think it's because my thumbnails are way too big. I'm not a Photoshop expert and I don't have a lot of money to spend on software right now. Does anyone have a list of good, free web tools where I can just upload an image and have it compress it down for the web? Need something that doesn't leave watermarks or ruin the quality.

TinyPNG is the gold standard for web compression. It strips all the unnecessary metadata and compresses the images without you even noticing a difference in quality. You can use it in the browser, or they have plugins for WP. There's also Squoosh, which is a web app made by Google. It gives you a bit more control over the output format.

If you have a lot of images to process at once, you can use Caesium. It's a free desktop app for Windows. You just drag and drop a whole folder and set your compression level. It saves a ton of time compared to uploading them one by one on a website.