change the message shown in the image upload dialog box

[alicewriteswrongs]

Pre-Flight checklist

• Testing
  • Code is tested
  • Changes have been manually tested

What are the relevant tickets?

closes #1127

What's this PR do?

This changes the text we display in the image upload dialog box, to match what is on the mockups (see the linked issue).

How should this be manually tested?

Go to one of the image upload things, and make sure that you see something along the lines of the mockups there.

[codecov-io]

Codecov Report

Merging #1338 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1338   +/-   ##
=======================================
  Coverage   96.48%   96.48%           
=======================================
  Files         417      417           
  Lines       15137    15137           
  Branches      440      440           
=======================================
  Hits        14605    14605           
  Misses        477      477           
  Partials       55       55

Impacted Files	Coverage Δ
static/js/components/ImageUploaderForm.js	100% <ø> (ø)	⬆️

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