It is generally useful to combine multiple pictures into single one. A example case, where merging pictures on Linux is necessary is if you previously used ImageMagick's convert command line tool to convert PDF file (pages) to JPEG / PNG pictures. Unfortunately convertion with convert(as far as I know is only capable of generating multiple picture files instead of one single one), thus you further need montage to merge pages in separate photos to one. In my case I had my Curriculum Vitae in PDF and I needed to have same PDF in single photo for my applications for online Job Employment Belarusian portal site rabota.tut.by.
montage is one of numerous ImageMagick package script (plugins).
On all major Linux distributions (Debian / Ubuntu, Fedora, CentOS, RHEL, SuSE) montage comes installed together with imagemagick deb / rpm package.
Whether you don't have montage on Debian / Ubuntu and deb derivatives install it via:
linux:~# apt-get install --yes imagemagick
On CentOS, Fedora, RHEL, SuSE to install montage:
[root@centos ~]# yum -y install imagemagick
To merge two JPEG Photos into single PNG format picture:
linux:~$ montage -geometry +2+2 Picture-1.jpeg Merged-picture.png
Combining more photos, lets say my 8 Pages photos output from previous PDF convert to pictures is done with:
linux:~$ montage -geometry +8+8 CV_Georgi_Georgiev_bg-0.png \ CV_Georgi_Georgiev_bg-1.png \ CV_Georgi_Georgiev_bg-2.png \ CV_Georgi_Georgiev_bg-3.png \ CV_Georgi_Georgiev_bg-4.png \ CV_Georgi_Georgiev_bg-5.png \ CV_Georgi_Georgiev_bg-6.png \ CV_Georgi_Georgiev_bg-7.png \ CV_Georgi_Georgiev_bg.png
montage has plenty of useful other options, to do various photo montages from command line. Other way to merge photos with montage is by using:
linux:~$ montage -mode concatenate -tile 1x input-pic*.jpg out.jpg
Merging photos is also possible by using directly convert.
Combining multiple photos into single JPEG or PNG with Imagick convert is done with:
linux:~$ convert -append input-pic-*.jpg combined-picture.jpg
Other example use of montage is located on ImageMagick's montage's script site here