Add comment for the supplementary action needed by template_tenprint_new_uploadimage() if no resolution is present in the image

Bug related to the commit 2626adc7

Use a more correct name for the parameter "table" instead of "db".

Move all the functions to a particular module in the utils folder,
avoiding circular imports and having a more clear view of the functions

The images for the tenprint template are empty, hence not encrypted in
the database. This patch add the selective decryption for images (only
submission images are decrypted)