DCIM thumbnails just stores the data of the image in order to make the picture viewing experience faster the next time you open it. Thus it is completely safe to delete DCIM thumbnails data on your device(or any storage media).
More about DCIM .thumbnails folder
Whenever you view the photos or videos on your phone, the thumbnail data will be automatically generated and will be stored in “.thumbnails” folder. For instance, when scrolling faster between various images, you may notice that it may take some considerable amount of time. However, when you scroll back (in reverse direction) the images wont take time to load (as they are already loaded with help of .thumbnails folder). So, deleting the .thumbnails folder would not affect the pictures stored on your device in any way. However, when you view the pictures/video files again it may take up some time to load. Further, even if you delete thumbnails, your device will automatically generate new ones when you have a look at pictures/videos.
Caution: Its is completely safe to delete thumbnails, but make sure not delete any important image/file along with them. If you have accidentally deleted your folder containing images then you can utilize Folder Recovery software(Remo Recover-Media Edition). The application is specifically developed to restore lost or deleted multi-media folders/files from your storage media. With help of this tool you can recover multi-media files with just a few clicks of the mouse.
Generally the photos are stored in DCIM folder to keep all the files in an organized manner. Along with DCIM folder, you came find .thumbnails folder as well. You can go ahead and delete the thumbnails but the images/videos take more time to load when you view them next time. However, when you delete the DCIM .thumbnail folder, some amount of storage space will be freed up