Visitors want the photos either because they need to identify their fish or just to enjoy looking at them. Your photos have to be of good quality. Blurred or out of focus shots should never be used unless temporarily till you come up with a better shot. A blurred photo will not serve any of the reasons it is published for. Bear in mind that a blurred or out of focus picture can't be edited to look like the picture of the century.
Software can work miracles only if the original picture is of acceptable quality.
The size of the photo is also important. They may save some space but they are of very limited importance. Big size is preferable but do not overdo it. A picture, which requires the visitor to scroll up and down and then, left and right to see it is not the best option. In this case, the visitor has a full view of the fish, plant or tank while he doesn't have to bother if there is something else in the missing right part.