How should different sized RAM be added to iMac?

I have a mid-2011 iMac which came with 4GB RAM (2x2GB). I just bought 8 GB (2x4GB) and added it to get a total of 12GB. The current configuration is as follows:

Given that the RAM is a different size, do I have the optimal locations for the different RAM chips? At the moment what I did was to simply add the 2 new sticks of RAM in the two empty slots.

That configuration should be perfectly fine. You can consult Apple's support documentation to be sure, but as long as your top two slots are filled using the original factory RAM and you installed the new RAM into the bottom slots (and you're using the right kind of RAM, of course) then your configuration should be OK.

Don't forget to run memtest!

I'm going to take WAGuess* and suggest that any gains (realized or not realized) due to memory-size matching will pale to insignificance beside the gains due to tripling your memory. My rule of thumb for processor speed is that our perceptions tend to be more logarithmic than linear and that we don't really notice a speed increase until it is 50% - 100% greater than what we had before.

If you're in a situation where a few - consistently acheived - percentage points matter, such as frequently or perpetually running long, compute-bound jobs, and and where finishing a job a few minutes earlier might have a significant benefit for someone, then there may be an advantage to optimizing your memory placements. Otherwise enjoy the speed of you newly augmented machine! You've already got 99++% of the benefits to be had.

'*' That guess isn't completely WA'd: I've already looked into using mis-matched memory sizes on two of my older Macs, and the above is what I concluded from at the time.