Revolution Slider – image layers are pixelated. Why?

My background picture is totally fine but when I add image layers on top of it they are pixelated. I tried redoing them in a vector format and saving them at 2000px by 2000px with 300px/inch and then resizing them in the slider back down to the 150px by 150 that I needed. They were still pixelated. I thought that maybe they were just that way because the computer was having trouble loading such large images so I tried making them 150 by 150 right in Photoshop but they were still pixelated. Is there any way to get around this? I had to save the pictures in a png format as the layers have a 75% opacity.