Image Convert

Make sure ImageMagick is installed or install it

ls -al /etc/ImageMagick-6/policy.xml
#if not present, install
sudo apt install imagemagick

Once you have it installed, use the ‘convert’ command-line tool of ImageMagic. Just provide the paths to individual images and at the end of the command, name the output file. For example, something like this:

convert image1.jpg image2.png image3.bmp output.pdf
#or resize by procentage
convert  -resize 50% source.png dest.jpg
#or resize to specific size
convert -resize 1024X768  source.png dest.jpg

If the following error appears:

convert: not authorized `pictures.pdf' @ error/constitute.c/WriteImage/1028.

Edit /etc/ImageMagick-6/policy.xml and change\ the PDF rights from none to read|write there. 

<policy domain="coder" rights="read|write" pattern="PDF" />