Right click on the image in your library panel.
There should be a checkbox in there for “Allow Smoothing”… uncheck that.
There is also another checkbox that says something about default compression or something, uncheck that and you can set your quality value where the higher the number the higher the quality.
As for format used…
If the image doesn’t have much colors in it, I use .gif either way. Since GIF is for best optimizing images with 256 colors or less. I also use gif for transparent images as many .png images tend to bog down system resources.
I use .jpg file types if the image I am importing uses more than 256 colors, such as a photo, or something with a lot of detail in it.
I use .png if my image has areas of partial transparency. Since .png images allow for grandient like transparency, it can add some nice effects. But this nice effect comes at a price with file size, and if that image moves, it comes with the price of using up system resources if you have too many on the screen (also depending on the size of them too).