what resolution (pixel/inch)are the photos at 2100x1500
for the web 72 pixel by inch is usually good enough, however I do show at 144 for photos I want to show crisp and sharp.
I would take them down to 1000x714.
if these 700 x 600 and 500 x 400 are setting then thats why its maybe pixelating the image cause the proportions are not right for your original size. you'd want 700x500 to make them in proportion if that size is ok for what you want.
hope that helps