28. A device, comprising: at least one hardware processor; and at least one non-transitory computer readable storage medium storing processor-executable instructions that, when executed by the at least one hardware processor, cause the at least one hardware processor to perform: transmitting, to at least one computer and via at least one network, a communication comprising a request to access a web resource; receiving, from the at least one computer and via the at least one network, an image that encodes a challenge text and software code, the image: comprising pixel values in a plurality of channels including a transparency channel, and storing decoding information for displaying the challenge text in the transparency channel, wherein the decoding information includes a plurality of saturation values for a respective plurality of pixels in the image; the software configured to use the decoding information to facilitate displaying of the challenge text by the device; executing the software code; displaying, to a user of the device, the challenge text using decoding information in the transparency channel of the image; receiving, from the user of the device, input specifying a response text; and transmitting, to the at least one computer and via the at least one network, a communication comprising the response text.
30. The device of claim 28 , wherein the software code comprises JAVASCRIPT software code for execution by an Internet browser executing on the device, and wherein the displaying comprises executing the software code using the Internet browser.