# Create an image mask with rounded corners
convert -size 100x75 xc:white -fill black -draw “roundrectangle 0,0, 100,75 15,15″ +negate mask.png
# Create a gradient (you may skip this step if you’re using an existing image)
convert -size 100x75 -fill orange gradient:black-none -sigmoidal-contrast 6,50% -colorize 100% 3.png
# Remove the rounded corners, using the mask
convert -size 100x75 mask.png -transparent white 3.png -compose minus -composite out3.png

This approach works, but the corners are a bit rough. There are more posts in this category with better solutions.