Gallery, being PHP, cannot do all of the complex image operations required to resize and otherwise manipulate images by itself. It requires other programs to be able to do this, and you must install these programs or Gallery cannot work.
NetPBM is easy to install yourself. You can grab the files from the Gallery Download Page. You do not need root access; just upload the NetPBM files via your FTP client (in binary mode) and CHMOD the directory and all the files in that directory with the following permissions: 755. If the configuration wizard can't locate the NetPBM directory it will ask you for the exact location and you can fill in the correct path.
NetPBM is not required if you are using ImageMagick.
ImageMagick is an alternative for NetPBM. It is usually considered to be faster, but can be more difficult to install. As with NetPBM, you don't need root access to install the binaries. You can download ImageMagick builds from http://www.imagemagick.org . Also as with NetPBM, the binaries must be chmodded 0755 (including the directory they are in).
ImageMagick is not required if you are using NetPBM.
If you want to display the EXIF information that many digital cameras store in JPEG images, you'll need to install a small program called Jhead to your webspace. A few hosts have begun having it pre-installed for Gallery users. If you are installing Jhead yourself you can install it either in the gallery directory or in your public_html directory (sometimes called www or something similar). If you don't need to display the exif information for your photos, skip this step.
Download the pre-built executable and upload it to your webspace via FTP, making sure that you upload it in BINARY mode. If Windows has saved it with a .txt extension, simply remove the extension once you've uploaded it and call the file jhead (with no file extension). CHMOD this file to 755. You will have to fill in the path to the Jhead file when you run the configuration wizard.
You can download the correct build for your webspace and find out more information here: http://www.sentex.net/~mwandel/jhead/
JpegTRAN support added in 1.4.0
JpegTRAN is a utility to allow Gallery perform lossless rotation on JPEG images. This results in a better quality rotation, as well as a speed increase, according to some.
JpegTRAN is incredibly easy to install, and is usually included in most distributions. JpegTRAN is usually included in the libjpeg or libjpeg-progs package. Simply install this package on your system, and (re)-run the configuration wizard and specify the path to JpegTRAN.
For Windows, JpegTRAN can be downloaded from the GNU Win32 project. It is in the jpeg package, specifically the jpeg-bin package. Download the file and unzip it in a directory (jpegtran will be in the bin/ directory), then specify the full path to it in the config wizard.