One Yahoo Group successfully transferred to groups.io has many
images in albums in the Photos section but also has some jpg
image files stored in the Files section; these are *only* jpg images
with no text.
Many groups have these, and it's a legacy from Yahoo at a time when they limited the size of images in Photos. The way around that limitation was to upload them to Files instead.
As I recall it was requested that a way be developed to move these images into Photos rather than download and reupload so as to not lose ownership. The alternative is to do all that manually and then reassign them to the correct owner if they are still with the group.
Dano
D R Stinson