RGB and CMYK and Web pages
It's not just a DNN thing, its a website thing.
Some browsers only know how to render RGB, since that's the format of displays. Others cater to print artists like Safari so they take the extra step to convert CMYK to RGB on the fly. Best practice: all web images in RGB.
Not sure what that means? In Photoshop and other photo editors, the image's mode tells it how the color is stored. Every pixel is assigned a string of numbers to store it's color. How about sprocket orange? It's RGB value is 255:125:25. It's CMYK is 0/63/99/0. If you tell Photoshop to save an orange picture in CMYK format, it stores 4 numbers per pixel. A browser opens the orange picture and expects to see 3 numbers per pixel, and so it gives up.
Set the mode to RGB and then save the file. Your browser will thank you.