You can convert multiple image (jpg/png/gif….) files to a single PDF in LinuxMint/Ubuntu by the imagemagick package. Here is step-by-step guideline:
 1. Install imagemagick from Ubuntu/LinuxMint Software Center or Synaptic Package Manager. You may not need to do this in LinuxMint as (if memory serves me right) it comes with the default installation
 If you prefer the terminal it's
sudo apt-get install imagemagick
 2. Put all your image files in a single directory.
 3. Open a terminal and go to this directory: cd 'directory-location'
4. Enter following command: 
 convert * output.pdf 
Where output.pdf is the desired single PDF file. 

Straight forward and simple. Thanks to Shafuil Azam for the heads-up