chore: add gray image test

[ykzts]

No description provided.

[codecov]

Codecov Report

Merging #171 into master will increase coverage by 1.52%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #171      +/-   ##
==========================================
+ Coverage   85.49%   87.02%   +1.52%     
==========================================
  Files           5        5              
  Lines         131      131              
==========================================
+ Hits          112      114       +2     
+ Misses         12        9       -3     
- Partials        7        8       +1

Impacted Files	Coverage Δ
convert.go	76.31% <100%> (+5.26%)	⬆️
transport.go	85.18% <100%> (ø)	⬆️

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