The best solution would be to not refer to the renditions directly. As a general rule the recommended best practice is to refer to DAM assets through a servlet rather than directly. This gives you better control of the URL and allows you better cache control in general.
So for example consider how the image rendering servlet in parbase works. It's image URLs look like /content/geometrixx/en/company/_jcr_content/par/3_1219086936652.img.jpg/1289480092880.jpg. The base servlet always returns the original rendition but it would be easy enough to add and additional selector into the pattern that was based on the rendition name and then write a rendition picker that selected the correct rendition.
Absent that you can use the out of the box thumbnail servlet that is available when directly referencing a DAM asset. It only works if your assets follow the pattering of being named based on their dimensions but as long as you are using the out of the box rendition creation functionality they should be compatible. The path to this approach looks like this: /content/dam/geometrixx/portraits/scott_reynolds.jpg.thumb.100.140.png. One key thing to keep in mind is that this servlet expects the image parameters to be in the reverse order of their rendition name. So with the example I just used the full path to the rendition is /content/dam/geometrixx/portraits/scott_reynolds.jpg/jcr:content/renditions/cq5dam.thumbnail.140.100.png. Notice how the order of the dimensions is reversed. /content/dam/geometrixx/portraits/scott_reynolds.jpg.thumb.140.100.png will return error (actually a generic icon).
There is one thing to be aware of when using the out of the box thumbnail servlet - it opens yo up to DDOS attacks since you can vary the dimensions infinitely and keep caching the generic icon under different names. This both puts you publish server under additional load and potentially uses up all your web server disk space. You either need to white list the dimension selectors you want in your dispatcher filter section, or write you own servlet with descriptive names to match the dimensions instead of actual dimensions.
Also another thing to be aware of - it's possible to create assets without an extension in them - if you follow the normal procedures for uploading an image it doesn't happen, however it should be noted that it's possible. For example look at this assets that ships with geometrixx sites - /content/dam/projects/media/cover - it's an image but the path does not include an extension. If you try just adding an extension you get an error - so relying on your DAM assets to have extensions in their name is not guaranteed to work. 99% of the time you are OK but you should keep in mind it's possible to have and asset without an extension in it's name. Better to follow the pattern of using a servlet to render you images instead of direct references.
