If your image appears to be blurred and pixelated then you can define the minimum size of the thumbnail as 600×600 px or try increasing the dimensions ( width and height ) of your Thumbnail size by going to Admin Panel > Setting > Media.
Also please regenerate your thumbnail after changing the image size by using the plugin given below for the best result:
My current setting is 150×150, what the gallery shows is 350×350.
Instead of increasing the thumbnail size I would prefer toning it down to about 250×250.
So is there a way I can prevent gallery from upsizing the thumbnails?
Is there a way I can increase the number of columns also?