What is the Best Way to Chunk Digits when Memorizing Large Numbers?

I'm more than halfway done with reassigning images to numbers 00 to 999, and put the system to the test while memorizing some driving directions today.

I'm still stuck on one problem: how to break up large numbers into chunks of three digits, when some of the chunks begin with 0 or 00?  I want each image to be made up of the same number of digits, otherwise I think it might be difficult to easily calculate something like "the digit in position #52 is a 6".

For example, with images from 00 to 999, the number, 345061234002395840029403920139940, would be broken up into chunks like this:
(345-06-123) (400-239-584) (00-294-03) (920-139-940)
each () set is a locus, and dashes separate the images.

In the example above, if I wanted to look up “digit #20", I wouldn't be able to go to the 7th locus and then move back one digit to find the 20th (if always grouped in threes, the 21st digit would always be the last digit in the 7th locus).

With images from 000 to 999, the number would be broken up like this, which allows easy calculation of the position of any digit:
(345-061-234) (002-395-840) (029-403-920) (139-940)

The problem with the latter method is that it would require an extra 100 images for the numbers 000 to 099. If I can't find a better solution soon, I'm just going to add 100 more images.

Quickly Building an Image Library

Coming up with 1000 images is a bit daunting, but I thought of easier way this time.  Wikipedia has lists for just about anything: mythological gods and heroes, artists, actors, politicians, and much more. Just scan down the lists, which are often alphabetical...

I think it's important to pick images that are pleasant or interesting to think about.  Otherwise certain numbers (or combinations of numbers) can become unpleasant. :)