For the visualforce page i would take this one:
<apex:image value="{!CASE(YourObject.CurrencyIsoCode,'EUR','/img/euro.gif',
'GBP','/img/gbpfund.gif',
'/img/default.gif')}"/>
Like Mohith say's, here is an example with static resource :)
<apex:image value="{!CASE(YourObject.CurrencyIsoCode,'EUR',URLFOR($Resource.MyRes,'img/eur.gif'),
'GBP',URLFOR($Resource.MyRes,'img/gbpfund.gif'),
URLFOR($Resource.MyRes,'img/default.gif'))}"/>
And for the Image field this:
IMAGE(CASE({!YourObject.CurrencyIsoCode}, 'EUR', '/img/euro.gif',
'GBP', '/img/gbpfund.gif',
'/img/default.gif'), 'image_label')