Where can I find a LEGO collection of minifigure photos?
Here are three options that have what you are looking for:

1) Brickset has the highest quality and best organized photographs of minifigs, but they are far from complete. For example, as of this writing, the gallery does not have any of the City theme figures. The collection is growing all the time. For example, here is their collection of Batman figures:



Pros: Free to browse. Very good quality. Well-organized.
Cons: Far from complete.

2) BrickLink has a fairly thorough catalog of almost every part and minifig ever made, and they have individual photos of most of the minifigures, for example:



Pros: It's free to browse. It has almost everyone.
Cons: It's inefficient, the photos aren't all high quality, and it may not have every figure. Organization and naming are not always obvious when looking for a specific figure.

3) The Unofficial Minifigure Catalog. This is a series of catalogs that attempt to document every minifigure ever produced. It's available as a printed book through Amazon (~70 USD), as a PDF e-book through Lulu (25 USD), and as a smartphone app. Here's a sample page, taken from Amazon's "Look Inside" feature:



Pros: High quality photos, thorough, well-organized.
Cons: Costs money. Harder to search and find specific photos. Takes effort to convert to individual digital photos if that is what you need.