No.

Why do you need this prefix?

but they are generated by the system now, not by you (even for FTP direct upload)

I think there is a misunderstanding (from you or me :) )

in Piwigo 2.4 there is only one version of each picture which is persistant : the full-size in galleries or upload

If you want to transfer your pictures from 2.3 (so without keep albums, comments, etc.. right ?) just copy the full-size (in pwg_high if you have one)

previously we had

+--Album
  +--pwg_high
  | |-mg_3663.png
  | |-mg_3664.png
  | \-mg_3665.png
  +--thumbnail
  | |-TN-mg_3663.png
  | |-TN-mg_3664.png
  | \-TN-mg_3665.png
  |--mg_3663.png
  |--mg_3664.png
  \--mg_3665.png

now we have

+--Album
  |--mg_3663.png
  |--mg_3664.png
  \--mg_3665.png

so you don't have to worry about thumbnails prefixes, just don't copy them

right :)