For the web, always export at 72 dpi resolution.
If you are exporting as jpg, note the compression slider. I usually place the compression slider ALMOST all the way to the right (i.e., minimum compression). You have to be very careful using Preview to crop the photo. If you just click "save", Preview will save with LOTS of compression and ruin the image. So, you have to do "save as" and fix the compression.
Alternately, you can export as png, and that always works out great, especially if you want transparent backgrounds.
I have found that Image Tricks works great in combination with OG because the OG exported images almost always need some cropping or editing after export.
Our maps, the clickable map on the home page and the title image on our site,
http://walki-talki.com, were produced by yours truly on Omnigraffle, as was the graphical navigation bar on the left side of
http://www.samuraivillage.com
Payam