1. A system, comprising: at least one computer; and at least one non-transitory computer-readable storage medium storing processor-executable instructions that, when executed by the at least one computer, cause the at least one computer to perform: receiving, via at least one network, a first communication from a device different from the at least one computer; identifying, from the first communication, a request to access a web resource; generating an image comprising pixel values in a plurality of channels including a transparency channel, the generating comprising encoding a challenge text in the image at least in part by: generating decoding information for displaying the challenge text, the decoding information including a plurality of saturation values for a respective plurality of pixels in the image; and storing the decoding information, including the plurality of saturation values, in the transparency channel of the image; transmitting, to the device and via the at least one network, the generated image and software code that, when executed by the device, uses the decoding information to facilitate displaying of the challenge text by the device; receiving, via the at least one network, a second communication from the device; identifying, from the second communication, a response text; and providing the device with access to the web resource based on a comparison between the challenge text and the response text.