Update landing page Icons

[cchristelis]

Problem

The landing page still has the old icons, please update.

Proposed Solution

Use the update logos from moqups for

• [x] Fish
• [x] Invertibrate
• [x] Algae

[cchristelis]

Perhaps let's hide the circles around the icons as well, while we don't have any data for them...

[Vries14]

Fixed in #174