If you use (from command line or within a script... if you need the script just ask ;) ) the command file thisimage.png, it will tell you what image format you actually have.... Then you can rename accordingly...

This is a sample of what I get with a file with incorrect file extension .png which is really a jpg file:

user@computer:$ file logo.png
logo.png: JPEG image data, JFIF standard 1.01 

Edit: Oh, ok, I was lazy before... Here's an script that will rename actual gif files that are named with .png extension to their correct extension .gif:

#!/bin/bash
for NAME in $(ls *.png); do

if [ "$(file $NAME|grep GIF)" ]; then

echo "Renaming ${NAME} to ${NAME%.*}.gif"
mv ${NAME} ${NAME%.*}.gif

fi

done

cat name.iso.001 name.iso.002 so on > newName.iso

does the job almost all the time. Otherwise there is utility called lxsplit

sudo apt-get install lxsplit

I never used lxsplit but I think the way you use it to join is:

lxsplit -j name.iso.001