How to find out what program ranger is using to preview a file

I have a transparent terminal that works with ranger's image preview. I use termite (terminal emulator) and compton (compositor) for transparency (a nice combo when you use i3wm).

Here's the result:

The only problems I had were with tmux (images are displayed, but not always at the right position and sometimes the terminal needs to be refreshed).

Note that the text and the image are also transparent since transparency is handled by compton. This only works if you don't add too much transparency, else you need a terminal emulator that handles transparency and works with this feature (I tried with terminator and I did not work for instance, same for urxvt). 85% opacity gives me some transparency and good readability, so this setup works for me.

Update

True transparency is supported once again in termite, but it suffers from the same issues as urxvt when you enable transparency (failed to execute w3mimgdisplay error, cf. @slm's answer).

I found the solution, at least for text files, the problem was in the highlighting... Ranger was trying to highlight long files... the workaround I found is shown in the following excerpt of ~/.config/ranger/scope.sh

#!/usr/bin/env sh

path="$1"    # Full path of the selected file
width="$2"   # Width of the preview pane (number of fitting characters)
height="$3"  # Height of the preview pane (number of fitting characters)
maxln=54    # Stop after $maxln lines.  Can be used like ls | head -n $maxln

# Find out something about the file:
mimetype=$(file --mime-type -Lb "$path")
extension=${path##*.}

try() { output=$(eval '"$@"'); }
dump() { echo "$output"; }
trim() { head -n "$maxln"; }
hl() { command head -n "$maxln" "$path" | highlight --syntax="$extension" --out-format=ansi; test $? = 0 -o $? = 141; }

case "$mimetype" in
    # Syntax highlight for text files:
    text/* | */xml)
        try hl && { dump | trim; exit 5; } || exit 2;;
esac
exit 1

The idea, it to select only the first lines of the textfile and then call highligh only on that portion.