I believe the problem is simply with the lines that say 'return [Vtec file].jpg not returning the path to the files.

Since the files are in the same directory as the script, we can use $scriptdir. Below should be the right code.

alias blue.top { background -l $qt($+($scriptdir,$blue.top.f)) }
alias blue.side { background -h $qt($+($scriptdir,$blue.side.f)) } 

As far as I can tell everything was fine, except that we weren't specifying the path to the files, so mIRC didn't know where to find them.