Some image files - original on the left, decoded BMP converted to PNG on the right.
The first JPG I've found on my PC (XP that time).
Baseling jpeg, Restart Markers.
Found here and successfully decoded.
One component only.
4 component. This is the original CMYK JPEG image from Wikipedia and the converted one.
K1.JPG is a progressive, 4-component JPEG, I've spent the most time with this one: