I'm not really sure about the 'fixedstrokexxxxxxxx' and sprite files, I'd imagine they were to do with the gallery, but keep the RAW files, they tend to be quite handy. If you have the time use a file explorer to check the sizes of these files. if they're particularly small, say 12-50kb)then they're thumbnails for the gallery. You can delete them but the system will just recreate them, normally in a separate folder though so maybe some update from sammy will fix it. Try viewing the file by adding '.png' at the end and see if they're low resolution, or if the file names bear any similarity to any pictures you have. Ultimately you can just copy them to your pc and see how and when the system recreates them.