Add official png versions svg files to img/logos

[joshmoore]

https://www.openmicroscopy.org/artwork/ only provides svgs for hot-linking. There are pngs specifically for twitter & facebook but not as part of the artwork files.

Also: list RGB colors for the logos:

• OME dark blue: #1C4A87
• OME red: #DF283F
• OME green: #128669
• OME light blue: #1D8DCD

[sbesson]

:+1: for adding those. We also had some requests to add the IDR logos to the artwork as well.

In terms of management, this might be a good time to extract and manage these artwork files outside this repository i.e. github.com/ome/logos or github.com/ome/artwork. The advantage of this extraction is that we can add content much more rapidly, use GitHub releases for generating downloadable bundles (all logos or potentially subset e.g. SVGs, PNGs) and also mint DOIs via Zenodo

[joshmoore]

Updated with color values.