If anyone is still having issues with this, like already mentioned I recommend bilinear filtering and setting the aspect ratio and resolution in the retroarch.cfg file. I am still tinkering with the PSX for me shaders are all rubbish, would love high res textures but doubt the pi can handle it
I play FF VII and someone one redid all the textures and models in the PC version and really does make the PSX version look like a Vic20 edition :D
The big advantage of using svg files over png though is that svg is vector graphics, because of this you only need one file and can resize it to what ever you want without loosing quality.