Why do photos look sharp on desktop and tablet but blurry on a smartphone?

Thanks for the site link, Mary - now I can see exactly what the problem is. It's really off-topic for this forum (it's a web development issue rather than a photography one) but I'm sure others may be interested in the answer too.

I see that on the iPhone I get the mobile version of your site. If I click on About it takes me to this page. And here's the main photo used on that page:

http://marycarrollphotography.com/userfiles/images/mob/7749983bbe7aa0ec48708da7576e5660b9a6dbbd13015216591112779103.jpg

(I hope you don't mind me including that here. I haven't uploaded a copy, I've used the direct URL to the image on your site.)

The image size is 280 x 385 pixels. That's a great size for the older generation of smartphones, such as the iPhone 3GS and earlier which had a screen resolution of 320 x 480 pixels. However, the iPhone 4 onwards - and other comparable smartphones using Android etc. - have much higher-resolution screens. The retina display of the iPhone 4 and 4S has a resolution of 640 x 960 pixels: although the screen is the same size as on earlier models the pixels are much smaller, giving a much sharper picture.

What you're seeing on your iPhone 4S is your 280x385 image scaled up to twice the original width and height. That leads it to look soft compared to a "native resolution" image. I see exactly the same on my iPhone 4: it's especially noticeable around the frame of your glasses.

The solution is definitely off-topic here, I'm afraid, but you need to see if you can publish a higher-resolution version of your site that will still work on older handsets.