I'm working on an image heavy website, so would really need to use a Photoshop like program to reduce file size. I came across Photopea - how legit is it? Can I use it for this job?
Photopea does the job. It replicates very useful subset of Photoshop's functionality for RGB images. Running it with a web browser in a distant web server makes the job a little slow. All your data you insert to the service can be investigated by the owner of Photopea and used in mining useful commercial information. Photopea doesn't directly claim any ownership of your data, but you should read the terms very carefully by yourself because I'm not a lawyer. I guess you shouldn't upload any images which are not yours and only yours.
Not asked, but maybe useful: There exists plenty of free photo editing software which run in your computer. For edits, format conversion, scaling and cropping you can use for ex. GIMP, Paint.NET, IrfanView and FastStone Image Viewer. The last two allow very easy batch scaling and format conversion if you like to handle say 100 images with one command. They are not handy for complex image edits.