Standardize on more easily divisible image ratios

[oubiwann]

From work done on:

• Ticket Explore mechanisms for storing, indexing, and retrieving raster data raster#25
• Spreadsheet https://docs.google.com/spreadsheets/d/1IBeVdiIy4LyOaXEWL3bd1EKOJaQ_87AwR16t5MwPd6M/edit?usp=sharing

we need to standardize on the following resolutions:

x	y
6144	4096
3072	2048
1536	1024
768	512
384	256
192	128
96	64
48	32
24	16
12	8
6	4
3	2

Image changes needed:

• 3600x2300 (large) → 3072x2048
• 1600x1022 (medium) → 1536x1024
• 500x319 (small) → 768x512
• 240x153 (tiny) → 384x256

Also, the README will need to be updated:

• Add a new "resoltions" section
• Populated it with the info above and some descriptive text, rationale, etc.