Load and close PIL images with context manager when making FIB GIF

[tieneupin]

Now that the FIB workflow has been deployed to production, we discovered that the PIL Image classes created as part of making the FIB GIFs were not closed properly, leading to too many files remaining open in memory.

This PR fixes that by ensuring that the images are opened and closed in PIL using the context manager.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.40%. Comparing base (28b4131) to head (28feb77).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #817   +/-   ##
=======================================
  Coverage   52.39%   52.40%           
=======================================
  Files          98       98           
  Lines       10491    10493    +2     
  Branches     1385     1384    -1     
=======================================
+ Hits         5497     5499    +2     
  Misses       4705     4705           
  Partials      289      289           

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.