Images in the download site are missing

[wenzeslaus]

The issue is likely coming from #550, but new approach is needed anyway. The logo can be used as a placeholder. However, the new images can be without the GRASS logo - that is implied, the logos here are for navigation. Something like:

[cwhite911]

I've just addressed this in PR #606. It is just using the previous image in the fix, but we can change them to newer versions later. Are these images already part of the sites assets?