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Try settings-apps-all-album,force stop and clear data.Restart the phone.
you can use a file manager and delete the thumbnail folder
internal storage/sd card > DCIM > thumbnails
Follow the upper suggestion by uli,report.
DCIM-.thumbnails.
if you stored the pictures on your SD card then
on a PC
computer > xperia L > sd card > DCIM > thumbnails
with a file manager
ext_sd card > DCIM > thumbnails
Anytime