Here's what I found out about imgur filenames:

For any Imgur image a typical URL and naming scheme would be:
https://i.imgur.com/XXXXXX.jpg = 4288x3216 px : large original with 4:3 aspect ratio.
https://i.imgur.com/XXXXXX‘‘‘h‘‘‘.jpg = 1024x768 px
https://i.imgur.com/XXXXXX‘‘‘g‘‘‘.jpg = 728x546 px
https://i.imgur.com/XXXXXX‘‘‘m‘‘‘.jpg = 320x240 px : 4archive uses this one as thumb
https://i.imgur.com/XXXXXX‘‘‘b‘‘‘.jpg = 156x160 px
https://i.imgur.com/XXXXXX‘‘‘s‘‘‘.jpg = 90x90 px

Which translates into archive links:
https://web.archive.org/web/0/https://i.imgur.com/XXXXXX.jpg
https://web.archive.org/web/*/https://i.imgur.com/XXXXXX*
https://archive.today/https://i.imgur.com/XXXXXX*
https://desuarchive.org/_/search/filename/XXXXXX*/order/asc/
Specifying exact file extension is not required, Imgur automatically corrects it when needed.