.png is the (only) correct type.
I never use subfolders and no uppercase filenames, can you try that? If that doesn't fix anything, can you share the name which you use to point to the images?
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#Steam AliasWL%Streak
- 1.279110.717+17
- 2.270144.652+2
- 3.558230.708+2
- 4.422.955+12
- 5.29647.863+2
- 6.8120.802-1
- 7.19078.709+4
- 8.20387.700-1
- 9.399200.666+8
- 10.212115.648+17