[  Debug ] Element:  SHOWICON
[  Debug ] Modify
[  Debug ]      command & arguments : replace(debug)('index_title' "http://images.nos.pt/")
[  Debug ]      Expression-1            : /storage/.kodi/epg_icons/
[  Debug ]      Expression-2            : channel
[  Debug ]      Element value before operation:
[  Debug ] http://images.nos.pt/611236_resized_352x198.jpg
[  Debug ]      Element value after operation:
[  Debug ] http://images.nos.pt/611236_resized_352x198.jpg
[  Debug ] Debugging information SiteIni
[  Debug ] Element:  SHOWICON


This:
<icon src="http://images.nos.pt/611236_resized_352x198.jpg" />



To this:
<icon src="/storage/.kodi/epg_icons/AMD/611236_resized_352x198.jpg" />

AMD is channel name:
<channel update="i" site="nos.pt" site_id="415" xmltv_id="AMCHD">AMC HD</channel>



Try this code but don't work.
showicon.modify {replace(debug)('index_title' "http://images.nos.pt/")|/storage/.kodi/epg_icons/|channel}