Intel or AMD?
As mentioned, you should be looking for a quad-core processor if you're a serious image editor. While dual-cores provide adequate performance for casual users editing small image files, those who want to edit images shot in RAW format will want the quickest processor their money can buy. Even a six-core processor could be an option, but we'll get in to that later.
When it comes to per-core and per-clock performance, Intel dominates AMD. There's hardly a thing that I can say to provide support for AMD's high-end products, as they are simply not competitive with Intel in this field. That's not to say that AMD processors will be terribly slow, but why buy a less powerful processor when a better option is available at the same price?
If you take our advice and go with an Intel product, you should look first at the Intel Core i5-2500. This mid-range quad-core processor is priced at $209 and is generally considered the price-performance sweet spot. Its clock speed of 3.3 GHz can dynamically overclock to 3.7 GHz when there is thermal overhead available. Although not as quick as the Core i7 processors, the increase in performance you receive from this point on does not scale well with the increase in price.
Speaking of Intel Core i7, there are six-core processors available from the company under the Core i7 brand name. However, these processors are based on the older Nehalem architecture. While quick, they're not much quicker than more modern Sandy Bridge quad-core processors like the Core i7-2600. Yet they are priced much, much higher. In other words, they are not a good value.
There is one case where an AMD processor may be a good choice, and that's if you're on a tight budget. AMD offers a quad-core processor called the Athlon II X4 631 for $89.99, and another called the Athlon II X4 640 for $99.99. Although the performance is far less than a Core i5, these are quad-cores, and they are adequete. Ideally a serious image editor should go for the best, but that's not always possible, and these are valid budget alternatives.