Fix img_format

[saimn]

Fix #407.
Closes #412.

[codecov]

Codecov Report

Merging #421 (a1ea4a5) into master (6d56b28) will increase coverage by 0.09%.
The diff coverage is 95.87%.

@@            Coverage Diff             @@
##           master     #421      +/-   ##
==========================================
+ Coverage   86.40%   86.49%   +0.09%     
==========================================
  Files          22       22              
  Lines        1787     1799      +12     
==========================================
+ Hits         1544     1556      +12     
  Misses        243      243              

Impacted Files	Coverage Δ
sigal/settings.py	97.87% <ø> (ø)
sigal/plugins/encrypt/encrypt.py	80.12% <80.00%> (ø)
sigal/gallery.py	90.59% <100.00%> (+0.41%)	⬆️
sigal/image.py	91.22% <100.00%> (+0.71%)	⬆️
sigal/plugins/extended_caching.py	81.25% <100.00%> (ø)
sigal/plugins/media_page.py	100.00% <100.00%> (ø)
sigal/video.py	89.00% <100.00%> (-2.35%)	⬇️

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