JFYI, here's my personal taste;
- For spells, I cut the image with 4:3 ratio, resize them into 512x384, then make it 512x512 with adding 42 pixels up and 86 pixels down. (or adding 64+64 then shift 22 up)
- For creatures, I cut the image with 3:4 ratio, resize them into 384x512, then make it 512x512 with adding 64 pixels left and right.
- NEVER STRETCH the image into the size. If it is in the wrong ratio, I rather cut them instead.
- In some extreme cases I cannot cut image without losing key features (face boobs asses...) I use several image fill-up tools to making it in ratio. Even Windows10 basic provided MSPaint3D provides quite well functionality for those fill-ups.