Move some methods from thumbnail.py to image_helper.py
Optimize image checking code in image_helper.py
Update get_image_size import
Remove get_image_size code from thumbnail.py
Add get_image_size code from thumbnail.py to image_helper.py
Remove generate generate_thumbnail code
Add generate_thumbnail from thumbnail.py to image_helper.py
Finalize thumbnail.py
Remove thumbnail.py, which is resolved into image_helper.py
Update code in image_helper.py