Add a screenshot

[samreid]

In attempt to work around phetsims/perennial#64 I'm copying the gravity force lab screenshot to assets/ for a build. But we will need to generate an accurate screenshot for this sim. I'm not sure if the sim is ready for that yet, so I'll mark this as high priority (so it isn't neglected) but leave unassigned for now.

[samreid]

We can remove the placeholder image if/when phetsims/perennial#64 is resolved

[arouinfar]

Let's go with something like this @phet-steele

Alternates:

[jessegreenberg]

@phet-steele @lmulhall-phet do you know why this is on hold? Maybe because the mass colors have changed since #24 (comment)?

[arouinfar]

@jessegreenberg my guess is that screenshot issues are generally put on-hold until a sim reaches RC, at which point there aren't likely to be any further design tweaks.

That said, I would still go with these screenshots, albeit taken from the latest version.

[ghost]

Yeah, we usually try to take screenshots right after we're done with the last round of RC.

[zepumph]

Yeah, we usually try to take screenshots right after we're done with the last round of RC.

We are around that time, as RC.2 is in QA right now. I'm going to remove the on-hold label, though it may not be necessary just this second.

[KatieWoe]

Screenshots updated using phetsims/qa#438. @arouinfar please close if you are happy with results.

[arouinfar]

Thanks @KatieWoe. Looks good.