When adding a camera to sensr.net you can adjust the sensitivity of the motion detection on a scale of 1-10, where 10 is most sensitive. This value defaults to 5, which works pretty well for most cameras. If you change the setting to make the motion detection more sensitive, then it you may end up storing a lot of redundant images.
Let's think about this for a second. At first you might think that more sensitive is better. After all, you don't want to miss that motion. However, most cameras have some degree of noise in the image. This means that at even if nothing is going on, the images may still differ significantly from one frame to the next. Most motion detection, including ours, deals with this kind of randomness. However, if you crank up the sensitivity, you'll still run into this issue.
Ideally, you'll want the motion detection to trigger when there is significant motion. In other words, when something interesting is going on. After all, you don't want to spend hours looking to lots of boring images. What you really want is to know when something interesting is happening or has happened.
There is another issue to keep in mind, we do impose quotas on the number of images that we'll store per hour. Right now we limit it to about 600 per hour. (It's a soft limit, so sometimes you'll get more...) This means that if your camera setting is too sensitive, you'll waste your quota on lots of almost identical images, and when that raccoon walks through the office, you might miss it.
If you're getting too many images (also known as false positives) from your sensr.net camera, click on the edit tab and adjust the senstitivy settings under advanced settings. I've found that for cheap cameras under low light conditions, the setting of 3 works pretty well.