I ran into the same issue earlier this week when I re-did my user lookup. The solution may or may not be simple, it just depends on what you need a picture of. If the item has been out for a while, like let's say a Normal Golden Nerkmid, getting the <img> source won't be too hard. Do a Shop Wizard search for "Normal Golden Nerkmid", but instead of searching in items for sale, click the radio button by "galleries" to search there instead. Click on any of the usernames you get from the search and look through their gallery until you find the Nerkmid. If you use Internet Explorer, right-click the image and select "properties". The properties window will pop up, then look at Address (URL) for the http:// link. There's the code you need. If you use Mozilla, just right-click the image and select "copy image location" then paste the link where you want the image to show up. Alternately for Mozilla versions over 2.0, the "properties" option also shows up when you right-click an image. I just like the ease of "copy image location".

For newer items, finding the <img> source might be a bit trickier, as folks might not have them up in galleries yet. The easy way around this is if the item you want is really new --look for its picture in the news and right-click on it there. I did that when I wanted to put a Gormball Necklace on my wishlist.

If you still can't find what you are looking for, and you truly need the image now (and you have some free time to waste) you can try "guessing" the <img> source yourself. How? Paste http://images.neopets.com/items/ into the address bar of your browser and guess away. Fun it's not, but it is a good last resort. Here's a tip:

If the item is wearable, it will have a "clo" in it, like this:
http://images.neopets.com/items/clo_gormball_medal.gif

Some names are easier to guess than others. The one above is for the Gormball Necklace, but the <img> source names it as a "medal". All I can say is: good luck!

I hope this answers your question.