Whenever an image is resized on Stack Exchange using URL modifiers, any transparent elements or backgrounds will be lost in the conversion.
Exactly how the missing transparency is handled can vary depending on the modifier being used, and the size of the image being converted.
When testing with a 1000x298 image:
l
,m
,t
: Transparent background was converted to blackb
,s
,?s=xyz
: Transparent background was converted to whiteh
: Image was not modified (it was smaller than 1024×1024)- HTML modifier: Transparency is preserved
When testing with a 5525x1275 image:
h
l
,m
,t
,b
,s
,?s=xyz
: Transparent background was converted to black- HTML modifier: Transparency is preserved
A similar issue involving profile pictures was marked as status-completed back in 2013, so it seems odd that this issue would still be around.
Original (1000x298): https://i.sstatic.net/s1dWm.png
Large Thumbnail (640x191): https://i.sstatic.net/s1dWml.png
Medium Thumbnail (320x95): https://i.sstatic.net/s1dWmm.png
Small Thumbnail (160x48): https://i.sstatic.net/s1dWmt.png
Big Square (160x160): https://i.sstatic.net/s1dWmb.png
Small Square (90x90): https://i.sstatic.net/s1dWms.png
?s=64 (64x64): https://i.sstatic.net/s1dWm.png?s=64
Custom HTML: <img src="https://i.sstatic.net/s1dWm.png" width="400">
-Thanks to Awesome Poodles, Emanuel Ve, and a cat for documenting imgur's URL modifiers