Allowed file types .webp size <20KB (20480 bytes)
Anchor point to string is ~ the middle of the left side so rotate the image counter-clockwise 90 degrees
Properly rotated resolution must be under width x height of 400 x 320 px

Example of easy steps to prepare an image for upload using a free software called GIMP:
Image > Crop to content (to get rid of useless px resolution)
Image > Transform > Rotate 90d counterclockwise (so it hangs in the mod from the upper part)
Image > Scale Image >change width to ~280x.. to lower the resolution for mod
File > Export As >change filename extension to .webp > export > modify the 2 quality % to around 70-80% for both

Then confirm the file size is under 20480 bytes limit
You need to fiddle with the last 2 operations until you're happy with resolution size vs quality vs file size
The more complex an image is, the more quality you have to sacrifice in order to respect resolution and file size