image base64 conversion

I have a signature field (drawing box) in CF7 that your plugin saves as base64. I need it to save as jpeg or png.

The CFDB7 plugin saves it as a .png file.

Without digging into your plugin files, I’ll ask here: can I do this with your plugin using an action or filter function? Is there a hook?

Also, every field is saved to db, including hidden and empty ones. Is there a way I can selectively save desired fields?
Having empty fields saved can result in bad aesthetics. For example, I’m building a school site with specific user roles allowed to view registration form submissions for approval/denial. This form provides for 5+ students, so there can be quite a lot of data if all are used. But, when only 1 or 2 students enroll, the superfluous, empty data of the unused toggled-out sections causes clutter of the form results page that these roles will view. They have to scroll way down from student #1 through the empty student fields to get to the bottom signature field.
This also makes printouts less attractive and wastes paper and ink/toner.