Here's a quick and dirty on installing the latest (version 6.5.0-0 as of this writing) ImageMagick on Ubuntu 8.04.

Start by removing any old versions previously installed via apt-get:

sudo apt-get remove imagemagick

Then update apt-get and install some supporting packages:

Use wget to grab the source from ImageMagick.org.

Once the source is downloaded, uncompress it:

tar -xzf ImageMagick.tar.gz

Now configure and make:

cd ImageMagick-6.5.0-0
./configure
sudo make
sudo make install

To avoid an error such as:

Add the following line to ~/.bashrc:

export LD_LIBRARY_PATH=/usr/local/lib

Update: If you still get an error like the one above, try running ldconfig:

sudo ldconfig

You can confirm the install and available formats with:

identify -list format