file.py

#!/usr/bin/python
#Download all sluggy freelance images

import sys, os
while True:
    mage = int(raw_input('image to download: '))
    mage += 1
    lett = 'a'
    imagetype = '.gif'
    os.system('wget http://www.sluggy.com/images/comics/'+mage+lett+imagetype)

Output

Traceback (most recent call last):
  File "/home/look/workspace/slug_lancer/src/down.py", line 11, in <module>
    os.system('wget http://www.sluggy.com/images/comics/'+mage+lett+imagetype)
TypeError: cannot concatenate 'str' and 'int' objects