It recently occurred to my I had completely forgotten about this thread. I'm so sorry I never came back to thank you all for your precious help.
With your advice I found out how to make things work the way I wanted them to. Basically, I need to run every single .jpeg file through Photoshop and resize it to precisely 600x800 pixels. Then I open Calibre and use the 'Edit Metadata' tool to change the cover. After that, I send the ePub file to the external SD card and it looks perfect! Every cover looks exactly like the third picture
GeoffR shared above.
There's probably an easy way to do this but I'm a bit of a technophobe. I mean, I love technology. Technology is great but at the same time I fear it! I fear I may mess things up and damage things for good. So as far as this "system" works I guess I'm sticking with it, doesn't matter how time-consuming it can get. Unless someone comes up with an equally easy solution...
Again, thank you so much for the help
