The code is working when I test it. One possible reasons why it would fail is that you have some other code that is replacing the load event that you set, for example by setting the onload attribute in the body tag.
Replacing the entire body content might conflict with other code that is manipulating the content. Preferrably you should only replace the content of specific elements inside the body.
I would rather use a regular expression to make a single replacement instead of looping through the strings and doing one replacement at a time. That should help a lot if you replace the content in specific elements instead of the entire body.
Most of the image tags are the same, so you don't have to repeat that for every card. The image should have an alt attribute to conform with the standards, you can use the original text for that.
window.onload = function(){
var cardname = {
'Ace of Hearts': '7/7c/Ornamental_h_a.svg/88px-Ornamental_h_a',
'2 of Hearts': '7/7c/Ornamental_h_2.svg/88px-Ornamental_h_2',
'3 of Hearts': '7/7c/Ornamental_h_3.svg/88px-Ornamental_h_3',
'4 of Hearts': '7/7c/Ornamental_h_4.svg/88px-Ornamental_h_4',
'5 of Hearts': '7/7c/Ornamental_h_5.svg/88px-Ornamental_h_5',
'6 of Hearts': '7/7c/Ornamental_h_6.svg/88px-Ornamental_h_6',
'7 of Hearts': '7/7c/Ornamental_h_7.svg/88px-Ornamental_h_7',
'8 of Hearts': '7/7c/Ornamental_h_8.svg/88px-Ornamental_h_8',
'9 of Hearts': '7/7c/Ornamental_h_9.svg/88px-Ornamental_h_9',
'10 of Hearts': '9/91/Ornamental_h_10.svg/88px-Ornamental_h_10',
'Jack of Hearts': '7/7c/Ornamental_h_j.svg/88px-Ornamental_h_j',
'Queen of Hearts': '7/7c/Ornamental_h_q.svg/88px-Ornamental_h_q',
'King of Hearts': '7/7c/Ornamental_h_k.svg/88px-Ornamental_h_k',
'Ace of Spades': '7/7c/Ornamental_s_a.svg/88px-Ornamental_s_a',
'2 of Spades': '7/7c/Ornamental_s_2.svg/88px-Ornamental_s_2',
'3 of Spades': '7/7c/Ornamental_s_3.svg/88px-Ornamental_s_3',
'4 of Spades': '7/7c/Ornamental_s_4.svg/88px-Ornamental_s_4',
'5 of Spades': '7/7c/Ornamental_s_5.svg/88px-Ornamental_s_5',
'6 of Spades': '7/7c/Ornamental_s_6.svg/88px-Ornamental_s_6',
'7 of Spades': '7/7c/Ornamental_s_7.svg/88px-Ornamental_s_7',
'8 of Spades': '7/7c/Ornamental_s_8.svg/88px-Ornamental_s_8',
'9 of Spades': '7/7c/Ornamental_s_9.svg/88px-Ornamental_s_9',
'10 of Spades': '7/7c/Ornamental_s_10.svg/88px-Ornamental_s_10',
'Jack of Spades': '7/78/Ornamental_s_j.svg/88px-Ornamental_s_j',
'Queen of Spades': '7/7c/Ornamental_s_q.svg/88px-Ornamental_s_q',
'King of Spades': '7/7c/Ornamental_s_k.svg/88px-Ornamental_s_k',
'Ace of Clubs': '7/7c/Ornamental_c_a.svg/88px-Ornamental_c_a',
'2 of Clubs': '7/7c/Ornamental_c_2.svg/88px-Ornamental_c_2',
'3 of Clubs': '7/7c/Ornamental_c_3.svg/88px-Ornamental_c_3',
'4 of Clubs': '7/7c/Ornamental_c_4.svg/88px-Ornamental_c_4',
'5 of Clubs': '7/7c/Ornamental_c_5.svg/88px-Ornamental_c_5',
'6 of Clubs': '7/7c/Ornamental_c_6.svg/88px-Ornamental_c_6',
'7 of Clubs': '7/7c/Ornamental_c_7.svg/88px-Ornamental_c_7',
'8 of Clubs': '7/7c/Ornamental_c_8.svg/88px-Ornamental_c_8',
'9 of Clubs': '7/7c/Ornamental_c_9.svg/88px-Ornamental_c_9',
'10 of Clubs': '7/7c/Ornamental_c_10.svg/88px-Ornamental_c_10',
'Jack of Clubs': '7/7c/Ornamental_c_j.svg/88px-Ornamental_c_j',
'Queen of Clubs': '7/7c/Ornamental_c_q.svg/88px-Ornamental_c_q',
'King of Clubs': '7/7c/Ornamental_c_k.svg/88px-Ornamental_c_k',
'Ace of Diamonds': '7/7c/Ornamental_d_a.svg/88px-Ornamental_d_a',
'2 of Diamonds': '7/7c/Ornamental_d_2.svg/88px-Ornamental_d_2',
'3 of Diamonds': '7/7c/Ornamental_d_3.svg/88px-Ornamental_d_3',
'4 of Diamonds': '7/7c/Ornamental_d_4.svg/88px-Ornamental_d_4',
'5 of Diamonds': '7/7c/Ornamental_d_5.svg/88px-Ornamental_d_5',
'6 of Diamonds': '7/7c/Ornamental_d_6.svg/88px-Ornamental_d_6',
'7 of Diamonds': '7/7c/Ornamental_d_7.svg/88px-Ornamental_d_7',
'8 of Diamonds': '7/7c/Ornamental_d_8.svg/88px-Ornamental_d_8',
'9 of Diamonds': '7/7c/Ornamental_d_9.svg/88px-Ornamental_d_9',
'10 of Diamonds': '7/7c/Ornamental_d_10.svg/88px-Ornamental_d_10',
'Jack of Diamonds': '7/7c/Ornamental_d_j.svg/88px-Ornamental_d_j',
'Queen of Diamonds': '7/7c/Ornamental_d_q.svg/88px-Ornamental_d_q',
'King of Diamonds': '7/7c/Ornamental_d_k.svg/88px-Ornamental_d_k'
};
document.body.innerHTML = document.body.innerHTML.replace(
/((?:Ace|[2-9]|10|Jack|Queen|King) of (?:Hearts|Spades|Clubs|Diamonds))/g,
function(m){
return '<img src="http://upload.wikimedia.org/wikipedia/commons/thumb/'+cardname[m]+'.svg.png" alt="'+m+'" />';
}
);
}
Edit:
To replace the contents of specific elements, you can for example use:
var elements = document.getElementsByTagName('DIV');
for (var i=0; i<elements.length; i++) {
elements[i].innerHTML = elements[i].innerHTML.replace( ...same as above... );
}
Edit 2:
As the tags only contains the card name, you don't need the replacement at all, you can just get the name and put the image in it's place:
var elements = document.getElementsByTagName('LI');
for (var i=0; i<elements.length; i++) {
if (elements[i].className && elements[i].className == 'card') {
var name = elements[i].innerHTML;
var img = cardname[name];
if (typeof(img) == "undefined") {
img = "8/89/Back07.svg/88px-Back07";
}
elements[i].innerHTML = '<img src="http://upload.wikimedia.org/wikipedia/commons/thumb/'+img+'.svg.png" alt="'+name+'" />';
}
}
