Screenshots

[KatieWoe]

For phetsims/qa#609. Screenshots need to be added before the sim can be published. Since the sim seems close to publication, I am marking this as blocking.

This was discussed on slack. @BLFiedler suggested something like this as the main screenshot:

Here are some possibilities for alt screenshots:

Please let me know if the main screenshot is acceptable, which screenshots should be alts, and leave suggestions if any are not acceptable. Once this is done the screenshots can be named and uploaded.

[terracoda]

These look good to me, and I like the main one.

[brettfiedler]

Thanks, @KatieWoe! I think these are all fine. If it's more appropriate to have 3, I'd eliminate Alt #1.

[KatieWoe]

Done in above commits. Closing