You can use find/xargs for this. It'll be faster because you can parallelize it with the -P option, for example, for a 4-core machine:
find . -iname "*.psd" -print0 | xargs -0 -P 4 -n 1 -I I convert I I.png
For best results, change the number "4" in the above command to the number of cores you have.
