Motion Detector Pro is a free Android app which uses your device camera to monitor an area. If any motion is detected then it immediately sends an email or text warning with a picture of the intruder.
This works well, though beware, it turns off if the phone locks. You could extend that time, of course, but it'll drain the battery, and whether you want to leave your phone around unlocked is another matter.
Setup is reasonably easy. Add a phone number or email address, depending how you want to be notified; tap Start Surveillance; position your camera so it's pointed at the area you'd like to guard, and tap "Activate Alarm". A ten second countdown gives you time to move out of the way, then the system is active.
A "Motion detector threshold" value allows you to tweak the app's sensitivity. The default setting of 50 worked for us, but if you have problems then try lower values to increase sensitivity, higher values to reduce it.
There are also many more advanced settings. You're able to tweak low-level details of the detection method, set the minimum time between triggers, decide where any grabbed images are stored (on your SD card, or in the cloud), even allow the app to be launched remotely by SMS (fortunately this option is password-protected).
What you don't get, unfortunately, is much control over the images themselves. No matter how good your camera, the pictures are taken at a low resolution, then heavily compressed, so don't expect to see very much.
We're not sure why your pictures have to be uploaded to the app's cloud storage, either. Why not just send them as an email attachment?
Motion Detector Pro only works when your phone is unlocked, and takes poor quality, low-resolution images. Despite that, it does have a solid motion detection algorithm, and if you only need to monitor an area for a short period of time then it might be useful.