I bet your browser has a web accelerator feature on it, and that you have that turned on.
These things work by reducing the resolution of the pictures so that the picture files are smaller, hence quicker to send. Unfortunately, lower resolution means blurrier. Usually if you right-click on the picture, it will open a little window that will offer you the option of "full resolution", or something like that. Clicking on that will clear up that picture. If you want them all clear all the time, you've got to turn off the accelerator.
If you don't have an accelerator running, then it's probably your monitor or the video driver.