Reliably changing the alt+mouse action depends on where it is set. What desktop environment are you running?
If you want to try outside the desktop environment settings, you might try xmodmap:
remove mod1 = Alt_L
remove mod4 = Super_L
add mod1 = Super_L
add mod4 = Alt_L
Then try it:
$ xmodmap "$HOME/.Xmodmap"
Otherwise, possibly change the key in Photoshop if that's possible? haven't had to use it in years; don't recall its key setting flexibility.