is resizing my images using lossy compression going to result in a quality loss?

Hey guys. I want to maximize my page speed and it appears more of my images need to be smushed. I don't see an option for lossless compression and every image in my wordpress has been smushed already. I'm considering doing the lossy comprssion smush but am concerned that if i do this and the image quality suffers, I will not be able to undo the action and reuploading them all over again is not an option. What are your recommendations?

Ross