I just use Irfanview... The easiest method I found does involve converting, but is very easy once you've done it the first time. It works well for me because I LOVE to take screenshots (perhaps too much)
once you have irfanview it will do batch conversions or re-sizing whatever. it also will save to a different folder if you wish or can overwrite within the input folder. I prefer to have FSX save the BMPs to the folder it uses and batch convert to JPG in a different folder. I can then just delete the BMP files in the original folder so it is empty for the next series of screenshots. This way you know it will not contain files that have already been converted to the preferred format. Irfanview will automatically save your conversion preferances so the next time you need to convert a batch you can simply select all the BMP files in the input folder and click "start batch" when it is finished you just delete the BMP files and you're ready for the next round. its quick and easy once you do the initial setup of preferances. This program is also good if you want to upload screenshots to a host that limits file size or pixel count. you can create a folder called uploadables, or thumbnails, and just convert a batch to a preset file size or dimension chosen by you. then the entire batch is exactly how you or the host site want it, and the process is fast and easy. And BTW, it converts the files very quickly, with no loss of image quality. (not that I've seen anyway..)