Online image samples

For Regular images 72 pixels per inch (ppi) is recommended.
For images with TEXT 300 ppi is recommended. 
Here are some sample sizes for reference:

12x18-72ppi; 864x1296 Pixels

12x18-300ppi ; 3600x5400 Pixels

20x8-72ppi ; 1512x576 Pixels

20x8-300ppi ; 6300x2400 Pixels

20x12-72ppi ; 1440x864 Pixels

20x12-300ppi ; 6000x3600 Pixels