It raises your fps but I don't think it will be a noteworthy amount.
Tested with 200 particles on an iPod Touch 5, 512x512 -> 128x128 increased the fps by ~1.5.
What has a much higher impact on your fps than the image size is how big the actual particles are but that wouldn't result in lower resolution particles but much smaller particles. Of course reducing the amount of particles is also a good way of gaining some fps.
Comments
It raises your fps but I don't think it will be a noteworthy amount.
Tested with 200 particles on an iPod Touch 5, 512x512 -> 128x128 increased the fps by ~1.5.
What has a much higher impact on your fps than the image size is how big the actual particles are but that wouldn't result in lower resolution particles but much smaller particles. Of course reducing the amount of particles is also a good way of gaining some fps.