After using your Android phone for so long, most people accumulate disk space in their .thumbnail folder. Deleting your .thumbnail folder every now and then is OK and will not cause any problems with your Android device.
In order to delete your thumbnail folder, connect your phone to your computer via USB. Then browse to your DCIM folder and locate the .thumbnail folder. In order to view this folder, you must set your folder viewing options to display hidden folders and files. If you don’t see the folder, click the top-right and select ‘Settings’ and untick ‘Hide system files’. Then go back to your DCIM folder. You should see the thumbnail folder. Go ahead and delete it. Feel free to restart your phone if you’d like. You should know that moving forward, your phone may run a bit slower when viewing photos since you need to regenerate thumbnails as you view them. No worries though, the amount of disk space you saved far outweighs the extra load time.