For me the pieces look fine, so perhaps it is a matter of refreshing your browser cache. I used 250 x 250 PNG images, copied from what you uploaded here without scaling but just a bit of padding and culling to make them all of equal size. Ad the Diagram instructs the browser to demagnify that to 50 x 50. (But you can zoom in without loss of resolution.)
For me the pieces look fine, so perhaps it is a matter of refreshing your browser cache. I used 250 x 250 PNG images, copied from what you uploaded here without scaling but just a bit of padding and culling to make them all of equal size. Ad the Diagram instructs the browser to demagnify that to 50 x 50. (But you can zoom in without loss of resolution.)