* Add selection ability using image query parameter
the selections is the index of the list of images,
where 0 is the first image. `os.listdir` is asumed
to always sort the list the same.
- if no selection is provided, take random image
from image collection (like main)
- if 1 selection is provided, return the nth image
from collection
- if 2 or more selections are provided, take random
item from the selection and return the image at
that index.
* Handle bad selection input
- Nondigit selections returns HTTP 401
- exclude indexes out of range (0--collection length)
* Re-randomize image if dimensions do not satisfy
* Try a maximum of 10 times to generate an image
* Decrease image generation cache to 1s
* Increase image quality
