If you go to your ACP -> General -> Board Configuration -> Attachment Settings. Now scroll down to [ Search for Imagemagick
You'll see this:
Full path to the imagemagick convert application, e.g. /usr/bin/.
If there is a file path in the text box, it's installed ok. If not, click the link and see what happens.
If there's still no file path. it's either not installed, not installed correctly or phpBB can't find it.
In that case, you'd have to seek support from their site to help get it installed. phpBB isn't for imagemagick support