How to generate a lot of (blank) images files?

The following should work for your needs:

#!/bin/bash
convert -size 1000x706 xc:white img_0.jpg || { printf '%s\n' 'Failed to create original image' ; exit 1 ; }
for (( _num = 1 ; _num < 1700 ; _num++ )); do
   cp img_0.jpg "img_${_num}.jpg" || { printf '%s\n' "Failed to copy to image img_${_num}.jpg" ; exit 2 ; }
done

ImageMagick creates the first image, and then it is copied to make up 1700 files. If ulimit doesn't restrict you from doing it (it probably will) and you have enough file descriptors, you could replace the loop with:

tee img_{1..1699}.jpg > /dev/null < img_0.jpg

You could also use the shorter idiom (with a larger memory requirement to store all of the numbers at once) for _num in {1..1699} in recent bash versions, but you don't specify which shells are available. (( is available in most shells (bash, ksh, and ash at least, not sure about others).