Your description is already too much technical...
Isn't it like this: you want to collect product data (name, description, price, image, etc) from users and then sell these products for them? Like Amazon?
Usually you would use an online database to store the data. This could be any online database, for example MySQL. The images could be stored on a web server and the links to the images in the database. Like this, there is no conversion to base64 and later no conversion from base64 to get the image back
Don't be afraid of security or hackers, first get it running, then make it secure.
Also at post 336 is is not too late to first spend some time into designing your app... well, it is probably a little bit late, but never too late.
This is why the goals of your app are important before starting to code... to not run into the wrong direction...
Take it as learning experience... and another suggestion... first learn the basics...
A very good way to learn App Inventor is to read the free Inventor's Manual here in the AI2 free online eBook App Inventor 2 Book: Create Your Own Android Apps ... the links are at the bottom of the Web page. The book 'teaches' users how to program with AI2 blocks.
There is a free programming course here Course In A Box and the aia files for the projects in the book are here: App Inventor 2 Book: Create Your Own Android Apps
How to do a lot of basic things with App Inventor are described here: How do you...? .
Also do the tutorials Our Tutorials! to learn the basics of App Inventor, then try something and follow the Top 5 Tips: How to learn App Inventor
Taifun
Trying to push the limits! Snippets, Tutorials and Extensions from Pura Vida Apps by 
Taifun.