Separate out the resize and caption functionality from upload functionality

[joyjit]

Currently, the script does these two operations every single time it is invoked

1. Resizes photos and applies caption to them
2. Uploads the resized photos.

Doing both operations every time seems wasteful. I will separate out these two functions

1. Run in "convert mode" --> converts (resizes and captions) ALL photos from source directory and places them in a (specified) destination directory. If a converted file already exists (check using filename only), then skip the conversion
2. Run in "upload mode" --> upload a set number of files from a specified directory as usual.

I will also preserve the original functinality