Since Tink seems to be offline, I'll say that the difference is that a slack pack is already compiled and has an install script - so it should be correctly compiled and be installed to the correct place. With a source compile, you have the opportunity to tailor it to your system and place it where you want - and also have the opportunity to screw it up.
Nothing ventured, nothing gained; the greater the risk, the greater the rewards and all that. But for something like imlib, yeah, just uninstall it, install from your disk, and then compile ice as normal. I think you could issue './configure --with-imlib=/root/imlib' and it might even work, but that's not really where you want it anyway.
And you could try recompiling/reinstalling imlib but might as well just do the slackpackage - it ordinarily would already be on your system anyway.