Free version in Google Play
Pro version in Google Play
There are already a bunch of apps in Google Play for reducing the size of photos for emailing. The one that I tried required many extra taps: select share, then select the app, then choose the reduce, then choose a resolution, then select share again. And it didn't respect photo orientations. Plus it had ads. I wanted something that doesn't require any extra taps over and beyond normal emailing. Plus something simple, small (35K!), and with no ads in ordinary use (the Options do have a link to the Pro version, but the Options doesn't show up in normal use, only when you want to customize the resolution).
There is source code (BSD license). It's very simple. From Gallery or Camera, when sharing a photo, just choose "Send Reduced" and it'll send it reduced to a size that fits in a 1024x1024 square (you can run SendReduced from your launcher to adjust this resolution). The first time you run it, you'll need to select which email app to use. There should be a checkmark to make this setting permanent, so that next time you just choose "Send Reduced" when sharing a photo it should just go straight through to your email app, and so it won't take any more taps than just sharing a photo normally.
You can also use SendReduced to attach a reduced photo. Just choose "Get Reduced Photo" when attaching (I only tested in gmail). Then choose Gallery as your picker, and set the checkmark so next time you won't be asked.
The Pro version lets you include some EXIF data and control the filenames of the reduced version. Try the free version before buying the pro, or just keep using the free version (or build the pro version from source, if fancy so takes you).
Needs Android 2.0.