I think it's because people just aren't used to seeing good quality pics here. How often do you get cell phone shots or kitchen pics. Ok, sometimes you don't have a choice but I think people can make a better effort if they want to share their pics.
Consider investing in a waterproof/shockproof digital camera like the Cannon D10 or the Pentax Optio W80/W90. I have the older W20 and it takes great pics too. Below is an underwater pic of a huge anemone.
You can also do yourself (and Sealine) a favour by creating an online album on a specialised site (like Google's Picasa Web Albums) to upload your pics. This allows you to insert multiple good quality pics into a single post (like above) using the IMG tag (Insert Image button at the top of the editor). It saves Sealine's disk space and bandwidth because the image is loaded from the other site (which makes Sealine load faster for everyone). It also allows people who aren't registered members on Sealine to view your pics.
There is of course the matter of taking a good quality pic, which means the person holding the camera needs to do a decent job. Lighting is an important factor: take the position of the sun into account. I tend to take (or request) a few shots each time to make sure I have a decent one in the batch. Shades help when the sun is shining in your eyes!
Taking good pics in low light or into the sun isn't easy, but standard digital cameras are getting better at it!
I love seeing good quality pics with the sea in the background! It not only shows the fish, but also gives an idea of the conditions you fished. So what if you leak a hint at the location? We're here on Sealine to share, right?
Good luck to all!